Crackling sound in Kdenlive...

Hello!

I'm trying kdenlive for the first time and I have a little problem...
I can edit ok video files in MJEPG (720p 30fps pcm sound 16kHz), but I have no joy with avchd lite files (720p 50i ac3 48Khz): the sound is crackling and I do not know where to start from for troubleshooting.

Is anyone able to help through a troubleshooting procedure?

I am using ubuntu 12.04 on a HP probook 5320m (the soundcard is an Intel IbexPeak HDMI); at the moment kdelive is the only software which is giving me problems...

Thank you for your help!