I'm pretty sure it is. However, I'd really reccomend against doing it. A lot of ppl use the mp3 player and his would interupt it making it sound bad. People just simply might not play the map because of it.
If you want the sound/song to repeat over and over again, you will have to set points in the .wav-file!
A year before, I wanted to add a repeating selfmade sound in a cs map too,
but everytime I came near the area where the sound should be played, HL crashed!
I asked for help in a forum and I was told to set points with goldwave!
These points show HL at what time the .wav should repeat.
I think, youve gotta add one point at the end of the wav, and another one at the beginning...but Im not sure!!!
You can download Goldwave for free, just use google to find it!