[greatoak]

I use my car audio to listen musics from my S4 phone via blutooth pairing. Depends on the music, the sound level differs by music to music, or even during the playback of one song, there is a phrase with low sound level or visc versa so that I have to adjust volume very often. Is there an app that automatically boost/suppress audio level to a preset level of my selection? Appreciate your help!

[Rukbat]

That's called a compander, and I haven't seen one for any phone, Android, Windows or iOS. I suppose it could be written - phones are certainly powerful enough. A compressor would kind of do the job - it would keep the high passages down, but it wouldn't boost the low parts. The question has been asked before (about a year ago here, and on a few other forums), but I haven't seen any compressor or compander apps yet.