As far as I know, it's an unfixed issue. A lot of people see it come up with televisions and stereos not playing sound. For me, this also affected the build mode music. It completely disappeared. I even noticed the sound effect for when you fast forward and pause the game had changed. There weren't as many 'clicky sounds' when it played.
In searching all over the net the day this happened to me, I couldn't fix my problem. Some suggested going to other lots and seeing that any playing stereos and televisions were off, but I don't get why this took away build mode music. Also, it's said that turning off the televisions and stereos is only a temporary fix, as the problem may strike again. I'll attempt this next time it hits me and post my results.
Maybe it only happens once in a while when a television/stereo is on in another lot, and this causes something to go wrong with the audio in general.
The solution that worked for me: Thankfully, in frustration, I saved, exited and restarted the Sims 3 and it fixed my problem. Your results may vary. All we can do is wait on the next patch to reliably fix the problem. Hopefully they're aware -- there's a 9 page thread at the official forum with no resolution here:
http://forum.thesims3.com/jforum/posts/list/90/2301.page