Sadly the FilmOn app has got worse with every so called update they release for it. I too have had this problem of no sound, according to users feedback it seems to be a common problem.
I managed to get mine working, by doing a none destructive reset, by holding down the Home and On/Off buttons down together, and don't release until screen goes blank and the Apple logo appears.
I may also have deleted it and re-installed it again...but certainly one of those fixed it for me.
It's rare I use it so much these days, as TvCatchup seems to work better, I've read the very latest update seems to have made it even worse again, as from feedback..users are saying a lot of the previous channels are no longer free, and is the reason I've not updated mine.
Try doing a reboot as mentioned above or maybe try a re-install...it seems to work for many users.
