My piano game : Mixing several WAV files result in extra white noise

I have been making a piano game which records my performance after playing.

It is powered by AndEngine , jcodec and an AAC Encoder Library

Here is the full source code: https://github.com/VansonLeung/PianoGameAndroid

However, the WAV sound mixing production results in extra white noise.

Did my algorithm go wrong?

The algorithm is inside: jp.classmethod.sample.audiomixer.AudioMixer.java

Thanks!