Before buying, please firstly try out the free version at https://play.google.com/store/apps/details?id=com.playwfd.avtools. After confirming that the free version has no problem on your phone, then decide whether to buy the Pro version. The difference between them is the free version can only access the output audio inside the application.

Audio Tools can be used to manipulate audio files:
1. Extract audio from a video file
2. Cut audio by setting a start and end point
3. Join/merge multiple audio files with fade in or fade out
4. Mix multiple audio files with different volume setting
5. Change audio rate or tempo
It supports the mainstream audio codec: mp3, aac, opus, speex, amr, wma, flac, etc. The output supports M4A/MP3/AMR codec with different compression level

For Chinese/English, it also can help on
1. ASR: automated speech recognition, convert audio/speech file content into text
2. TTS: text to speech, convert a text content into MP3 audio file directly