Play video+audio on incoming call? (+)

Hi,
I am now developing an app that is supposed to play video on incoming call. I managed to make it work pretty well on symbian 8.0 (N6680), but I am facing problems on 7.0s.
The probem on 7.0s is that I can see the video playing but I have no audio (or the default ringtone if any installed). I feel like I am missing smth since I know a lot of apps like mine that work well on 7.0s. So, what is the trick to override the default ringtone and make the video play with audio.
Thanks!