Can't export sound

Question

I have a video and audio track in a new project.  It looks/sounds fine in Premiere but once I export, the audio isn't there.  Instead I hear this loud buzzing almost screeching sound periodically.  There's a buzz...silence...buzz...silence that are a few seconds apart.  The audio file is an m4a, a song I purchased from iTunes.  Here are my export settings:
Source: 1920x1080
Output 720x480
Why isn't it outputting to 1920x1080p?
Export Settings:
Format: QuickTime
Preset: NTSC DV Widescreen 24p
Export Video checked
Export Audio checked
48000 Hz, Stereo, 16 bit
DV/NTSC 24p
I also converted the m4a audio file to an MP3 and added it to the project.  I clicked the eye on the m4a audio track so it doesn't play.  Only the video track and the MP3 track were available at that point and played in Premiere.  However, once I exported, only the audio played in the MOV file, no video. There was a black screen.  It was also a huge file - 1.34GB.
Any ideas what I'm doing wrong?
Here's my setup:
Production Premium 4
Macbook 2Ghz Core 2 Duo, 4GB RAM
Once last thing, the video doesn't render to well within Premiere.  It's jerky, doesn't render all frames and sometimes can sit on one frame for a while.  Is that because of my machine resources?  What specifically - processor?

Answer 1

Now, MP3 files can often cause problems. They are also highly compressed. I find them to be poor choices on many fronts.
It appears that you are going from an iTunes piece. Is this correct?
Now, I am on a PC, so there will likely be differences here, but this is my workflow from iTunes:
1.) create Playlist in iTunes with the necessary music
2.) Burn that Playlist to CD
3.) Rip the CDA material from the CD as WAV (Uncompressed). This will be 44.1 KHz 16-bit. From my audio editor (where I do the ripping), Audition in my case, I Save_As PCM/WAV 48KHz 16-bit.
4.) Import that file into PrPro for use.
Note: you might want to use another, non-compressed Mac audio format. I do not know how well, or poorly, Mac's work with WAV files.
I've never encountered the behavior that you describe.
Good luck,
Hunt