Hey there, if all you are trying to do is make an event that lets the user dynamically change the music it is actually really simple.
If you follow the example above, just give them a first choice option (the first red circle) that asks if they want to change the music. Once they select yes, you can then give them a list of all the songs they can change to.
If instead you wanted it to be a random song that can be easily done too in a similar style,
As above, you set a random variable and then simply do a series of conditional branches that check against the values of the variables and play the declared song.
The above images shows how you would set the random variable, simply go to "Control Variable" and select random and your range.
(This was done in VX, if your using Ace I'd imagine it would be very similar, but if your having any more problems, or I didn't understand what you needed, then I'll try to help further after you post again.)
<Alex>