IMVU disabled trigger-less music after version 309.
You can make it; but it will only play for 20 seconds at most, if it plays at all.
There are some types of triggered music that still work, but IMVU is working on getting rid of those too.
You can still make streaming music thou if you are willing to learn flash and actionscritp 3.
Sorry for the bad news, maybe someone else here can help you more.