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.

https://github.com/harrypm/DomesdayDuplicator

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.

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:

https://github.com/neonbjb/tortoise-tts

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.

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)

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.

You can explore more models on the huggingface page https://huggingface.co/models?pipeline_tag=text-to-speech&sort=trending

Most have a huggingface space dedicated to them where you can try them, here is the xtts space for example https://huggingface.co/spaces/coqui/xtts

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.

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).

There's a lot of results for ai voice clone. I can't personally speak to the effectiveness of them but it might be worth a look.

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.

Clone his vocal chords and get surgery to replace your old voice chords with the new cloned one.