Bad audio using MixMonitor?

Hi guys!
I was recording all outbound calls using MixMonitor() in .gsm format but when i play each file using the VLC Player, the file is playing really bad, i cant understand anything! For now i’m recording in wav format but this format have files are really big!!
I want to use .gsm format to save storage, please what to do or what player use to play the .gsm files (I’m using VLC player)
I have this in my dialplan