Since upgrading to Kubuntu 10.10 the sound quality of my renderings has been very poor, with chirping and other artifacts similar to what is discussed in this older thread: http://kdenlive.org/forum/x264-profiles-aac-chirps-poor-audio-quailty

I have installed the libavcodec-extra-52 and -unstripped-52, if that helps any. Not really sure where to start looking for the solution.

I am using the default HD 720p 29.97 profiles. Here is a YouTube video with an example of the poor quality audio:

http://www.youtube.com/watch?v=0RedODHDHQA

Does your libavcodec version provide libfaac? Run the config wizard in settings, look for the codecs tab, and look for libfaac - not 'aac'. If so, see if using libfaac helps by making a custom render preset from your preferred preset and changing acodec=aac to acodec=libfaac.
Alternatively, see if increasing the bitrate helps, again with a custom render preset and increasing the 'ab' option (try doubling it).

If neither of these help, then maybe you have a version of libavcodec installed that is not compatible with your version of mlt.

Thanks for your reply. I dug around in my repositories and added medibuntu, which replaced my libavcodec-extra-52 and libavcodec-unstripped-52 as well as some others, I re-encoded and the sound quality is fine now.