The Source html (which ULX uses) allows for embedded streaming windows media player content.
URadio (OLD no longer working release from gmod9 days) used these type of pages.
Figuring out the local relative path to play a file that was forcedownloaded however would be quite tricky.
ULX has force downloads of course by default, but combining the two would be a challenge.
I _really_ need to pick Umotd back up.
Though no plans for exactly what you're wanting, I did have plans for a 'radio' page. One of the community members actually edited it themself and added one. Take a look at the last few pages of the UMotd thread for the discussion.