There are very specialized filters for extreme stretching, e.g. ‘PaulStretch’ is popular among the ‘800%’ stretch-meme crowd. But it adds a distinct ethereal quality. Audacity seems to have this filter, and might be alright at stretching in general.

Command-line `sox` also does slowing down better than ffmpeg at speeds like 0.7, but dunno about 0.3.

Interesting. Just tried PaulStretch.

While it sounds more "logical" without the strange micro-repeats, it takes all of the punch out of the music.

On youtube, at x0.25 a drum hit goes from KABOUM! to something like:

KAKAKABOBOBOBUMUMUM

While with PaulStretch, it goes to something like:

FFHHAHAHCHCHOHUHMHH

No kick, no pressure behind the lower frequencies anymore.

It sounds like it is far far in the distance.

Sounds about right, that's why PaulStretch is typically used to create otherworldly ‘ambient’ soundscapes.

Something like Brian Eno turned back into proper Eno: https://www.youtube.com/watch?v=BnoX3E2WFcc