GitHub doesn't support adding audio files (.mp3 / .wav) neither directly to ticket nor to wiki page:
The first one (ticket) remains unsolved to me. The second one (wiki) was worked-around by:
- Pulling wiki repo's master locally
- Adding .mp3 files to main Wiki's repo (URLs doesn't work when using subfolders)
- Adding
[somefile.mp3](somefile.mp3)to some .md file - Committing and pushing back
Is there a better / easier way of achieving this?

The solution is as simple as -- zip it!
Since uploading
.zipfiles directly to GitHub tickets and GitHub Wiki pages is allowed then there's seems to be no better or faster way than attaching necessary files (of any format) inside.ziparchive.This solves both ticket-issue and wiki-issue mentioned by OP (me).
In case of .wav audio files mentioned in the question, compression also reduces the size of repository as these files are totally uncompressed by their nature.