I've been beating my head on the desk over some issues since upgrading to karmic and the kdenlive 0.7.6 a few weeks ago. I found that I was getting ffmpeg/melt/kdenlive crashes with pulsaudio installed. This was solved by removing pulseaudio and its config files.
I've now got a good working system and wanted to share a few notes with the hope that it helps someone.
Of course kdenlive recommends that pulseaudio be removed here: http://kdenlive.org/user-manual/troubleshooting-and-common-problems/soun...
In short I did this to remove pulseaudio. Follow Part A. Step 1 of this guide:
(Be careful when making these changes and understand what you are doing)
You can then use synaptic to remove some of the extra pulseaudio packages such as pulseaudio, and paprefs. Leave the libpulse stuff or it will want to remove too many useful programs.
I also had to go into the kdenlive settings and choose alsa as the audio device and the restart kdenlive.
This was hard to diagnose since most of the problems were observed in ffplay, which kdenlive uses. I had the most problems with files like .m2t that use a lot more processor and figured it was a problem with these files (so went down the path of upgrading ffmpeg). I did eventually settle on the latest x264 and ffmpeg svn, but the problem was only fixed by removing pulseaudio.
-In the past I also noticed jerky video and sound crackling in the preview window, which a lot of posts seem to mention also. Removing pulse also fixed that for me.
Hope this helps, Geoff