I have been using Audacity 2.0.0 without many problems. However, I just posted an mp3 to SoundCloud and, when I listened to it, I realized that all but two tracks cut out about 20 seconds into the song. I went back and listened to it on Audacity and it sounded fine. I then exported it again as an mp3, but again, all but the rhythm guitar and backing vocals disappeared 20 seconds or so into the song. I repeated this process a couple more times to no avail. Now I am completely stumped. Does anyone have any idea how I might fix this problem? 
This is where we get all compulsive on you. Exactly when does it drop out and does it always do it in exactly the same place.
Can you export a WAV? Microsoft WAV 16-bit PCM.
You know they’re gone locally by opening the MP3 up in Audacity? Windows Media?? Both?
without many problems.
Without any except for this one?
Save the show as a Project. File > Save Project As… and give it a name. That should let you open it up again just as you left it.
Now delete one of the multiple tracks. Does the export fail in a different point now? Does it stop failing?
Koz
Actually, there were three tracks on each of the first three mp3’s I created: backing vox, rhythm guitar, and lead guitar. I don’t think any sound dropped out after 20 seconds like I originally stated. That’s just when I noticed that it sounded really weird (Yes, I’m the drummer and I didn’t even realize that the drums weren’t there). So, the problem is that there are only three tracks that make it to the mp3. I deleted an additional guitar, but still got the same result after I made a new mp3. What is going on?
Thanks for your help!
Does the problem also occur if you export as WAV?
Let’s confirm the obvious before we go for the esoteric. Are all the Tracks NOT Muted and NOT Solo’d? Are their Gain sliders at zero (or some other level that will give an audible signal)? Are they panned to their proper positions? Can you hear all tracks when you play the project back through your speakers or headphones? If you cannot hear everything on playback in Audacity you have a different problem to hearing everything in Audacity playback but not in MP3 playback.
That was it! Some of the tracks were muted. I didn’t realize that they could be muted and still play in Audacity. Anyway, that worked. Thank you all for your help. 
I don’t think that they can 
If I mute a track in a multi-track project, the track is not heard.
That’s how it has always worked for me, ever since I started using Audacity (v1.2.6).
There was a bug in one of the early 1.3 Betas whereby muted tracks were Exported (but not played in Audacity) - but this bug was fixed a long time ago
WC