From time to time I got the need to trim intros/outros or silence from downloaded audio files.
I’m currently using Super Sound, but I find it a little silly that one of the last closed source, filled with ads app I still have installed is for such a small functionality.
On desktop I use ffmpeg for pretty much anything, so if there is some wrapper for it over android it would be ideal.
If you only want to trim MP3 or AAC files I can recommend mp3DirectCut. Ad-free, fast and simple.
PlayStore Link mp3DirectCut