Problem With MP3 Files Not Playing

Set it to 0 instead of playing 999 times, as in loop zero times, it should loop forever until something makes it stop.

In your event, open the Browse window, select the file you want and then click "Play" (instead of Open). If there are no sound, then there is an error in your file. If this is the case, I would download Audacity and export the file as *.ogg, as in my experience this is a way more stable format for Fusion. Here you can also remove the "white space" at the beginning and end of the track, which will make a smoother sound flow.
If the Browser window is able to play the track, then i would double check the event and make sure its actually happening. Next I would probably announce to the forum how you try to play the file (what kind of event, and if you try to play it as sample or music, etc).

Just going to echo what Kentronisk said

I've found that converting to ogg solves this issue

Bonus Tip

You mention your playing a sound at start of frame.

You would want to put in a check to make sure the sound isn't already playing as you will start a new instance of it every time that frame is returned to in your game.

2 conditions in your event
Start of frame
+
"X" isn't playing