Many streamers play music in the background when streaming (which I never like lol) and due to the copyright bs some portions of the audio are removed.
If you take streaming seriously and want to work around that, I have the needed solution.
What you have to do is to change the bitrate of the songs you prefer playing, especially popular ones that have a high chance of being recognized by the twitch algorithms and thus probably getting that portion muted.
You can do that easily using the freeware Audacity: http://audacityteam.org/download/
Reduce or increase the bitrate by 200 and you'll be all good.
Make sure to export the project not save it to get the files.
If you take streaming seriously you should think of doing this if you notice that twitch is removing audio from your videos.