You cannot embed music on your pages. This accidentally became "possible" when we opened up MOCpages to the inclusion of embedded video in Summer 2008. The hole was plugged after only a few days, but in that time a few pages with music were added by some people.
We don't plan on adding support for embedded music. Allowing you to embed apps opens up enormous vulnerability for serious hacking... MOCpages has been hacked by porn people and bulk mailers in the past, (which ended up costing Sean a few thousand dollars, by the way) and it wouldn't be hard for someone to embed a program that does really bad things, like redirect you to porn sites or steal all your personal information.
While some people feel that background music is a nice touch, others find auto-playing video and audio invasive and irritating. And the bottom line is that it's not really relevant unless it's a LEGO podcast or something, which you can always link to anyway.