It looks like two different problems:
- The audio can't seem to keep up with the video
This is rather a problem of synchronicity and you should check audio-video codecs or it's just a problem with speed of your internet connection
- Whenever I use the speakers (particularly for iTunes, music and youtube) they crackle every few seconds
Also - maybe a problem with speed of your internet connection (too slow)
One question - what's about plain audio (ex. mp3 file) - is it OK?