While tools exist, like people already commented, remember that the result may not be what you expect.
A recreation whether by AI or a skilled voice actor will have slightly different intonations, emphasis, tempo variations, pauses and lack of pauses that are not your granfather's. It is very likely to feel flat and wrong in an unpleasant way.
I can't speak to the AI voice generation part of this, but you might be interested in the Domesday Duplicator for digitizing your audio, especially if some or it is slightly degraded.
The project was originally designed for laserdisc, but it's been expanded to support VHS and cassette tape. Traditionally, you would play your tape on a cassette player, then the built in analog circuitry would convert the magnetic signals into audio, amplify them, and feed them to a sound card on your PC, which then converts the analog signal to a digital audio stream.
With the Domesdsy Duplicator, you record the raw magnetic signal from the read head and directly digitize it into a bitstream that you can then process as needed. For DIY archiving from an analog source, it's one of the best options for signal fidelity, and it will give you the truest representation of what's actually on the tape.
Maybe the term you are searching for is "AI voice cloning". The engine of https://elevenlabs.io/voice-cloning claims to be able to understand and reproduce even Danish.
Edit: They seem to require some voice verification to make sure the voice is yours. Which is odd in your case.
https://speechify.com/da should allow to recreate the voice of "your beloved one", at least they mention it on their German page.
I did sign up for ElevenLabs, unfortunately they cannot allow me to clone a dead persons voice, as per their FAQ:
You may only clone your own voice or a voice you have the rights to clone. For added security, when creating a Professional Voice Clone we require users to complete a Voice Captcha mechanism by reading a text prompt within a specific time to confirm your voice matches the training samples you upload for training. If there’s a match, your request is sent for fine-tuning. If not, you’ll have to reach out via our help center to have your voice verified manually.
Now I'm sure it wouldn't be an issue to get the legal rights, but when I spoke to their support, they did not have any way to verify beyond the captcha.
If you have the equipment (mainly an Nvidia GPU that has the ability) doing voice cloning locally is the way to go if you keep running into legal issues. Plus being on your computer you may be able to tweak and try different methods to get the best results for your needs. A year ago this would have been a maybe, but there's a lot out there to look at and try. See what others have done first in videos and such and follow their lead.
The very first thing you should do is get them professionally digitized, that way the quality won’t degrade any further. Then you can try training a voice AI, but as long as you have the digitized version, you can always train whatever new AI is invented in the future.
I've been able to generate very good results with this open source project. You need a pretty good nVidia GPU, and it takes some time and tedious work to get it working they way you want it to:
Some voices sound exactly right. Other sound like a broken robot. The main reason I like it is that I can run it local without having to sign up for some stupid cloud service.
I have only used it with American English. Oddly, it will sometimes slip into a British accent. I believe it is possible to retrain it on other languages, but I have not done the deep dive required to do so.
If you can get them into a digital format I’ve personally used eleven labs to clone voices and make narrations for missions I created for a video game. I tried using different open source projects and getting it to run on my own with no avail, but 11 labs has been solid (it is unfortunately paid software of like $5/10 bucks a month though)
Voice cloning is still kinda early, especially in the open source area. Whatever you may find now or later (it's a rapidly developing field), the first and most important step you should do is to digitalize the cassettes into actual audio files to prevent further degradation / loss. Make sure to back those files up too, preferably in the cloud I guess.
You should not need anything special to digitalize the tapes either. A simple cassette player with a 3.5mm audio output fed into your mic input on your computer and a recording software such as Tenacity should be enough (start recording on the program, then play the cassette).
Elvenlabs is currently the best but you can get some very good results with first xtts then rvc as a second pass. It involves fine tuning models and running things with python and notebooks, so requires some know how.
The language adds an other layer of difficulty, I would try their demo first to see if it gives anything workable but it isn't a language current tts software cater too, it doesn't seem to be an available option on xtts sadly.
Thank you for the tips. As I see it currently, I expect the language to be the biggest hurdle. It doesn't appear like something I can add myself, even if I had the data for a model. So as far as I can tell it involves two currently more or less impossible steps: Get model data and teach language to model.
If you have material with him speaking in English, you might be able to train an xtts model on it and then use that to bypass the elvenlabs captcha but I'm not sure if they give enough time. Although GPU rental is cheap these days, so captcha time is less of a factor.
If anything, the tech is moving quite fast, it will definitely be easier in a few years, maybe even months.
In the future, there should be more and more software to support vocal simulation. Although the functions are still relatively basic at present, the iterative version will support more functions, including the adjustment of speech speed/intonation. the new industry of DVDFab mentioned that their new product line - BookFab will update in the future. BookFab AudioBook Creator V3.0 will support custom voice cloning, with upgrades for multilingual adaptation and emotional expression. Additionally, it will include a simple audio editor for audio editing.https://www.dvdfab.cn/audiobook-creator.htm But we still cannot verify it, just wait and see. Hope you can find a great sollution.