Original audio versus library music
Original audio, meaning something the creator recorded, is theirs. Saving it for personal use is ordinary, and using it in your own work is a conversation to have with them rather than a legal minefield.
Library music is licensed to TikTok for playback inside TikTok. That licence does not travel with the file. Extract a trending track, put it under a video on YouTube or Instagram, and the automated content matching systems on those platforms will very likely flag it, regardless of the fact that the download itself worked perfectly.
Telling the two apart is usually easy. If the sound name is an artist and track title, it is library music. If it is a username followed by original sound, it is not.
What quality to expect
TikTok audio is AAC, typically around 128 kbps, sometimes lower for older posts. That is fine for voice, adequate for music, and noticeably compressed if you listen closely to anything with cymbals or reverb on decent headphones.
It is also often the third or fourth generation of a compressed file: recorded, processed by an editor, encoded by TikTok, and sometimes recycled from another platform before any of that. Do not expect a clean master. If you need one, find the track through the artist rather than through a video platform.
Practical uses that hold up
Keeping a voiceover for reference, saving an original sound a friend made, archiving your own audio from a video you posted, or grabbing a spoken clip you want to transcribe are all sensible and uncontroversial.
Building a music collection out of TikTok is neither, and the quality makes it a poor idea even setting the ethics aside. A 128 kbps clip of a chorus, cut at both ends, is a bad version of something you could stream properly in better quality.
Everything above also applies to the rest of TikTok video downloads, and our guide to silent downloads and missing audio goes further on the parts people get stuck on.