Have you considered using FMod? Its an interactive music stem player, that has "some kind of implementation" in Unity but not sure what... I believe you need a plugin.
http://www.squaretangle.com/FMODUnity.htmlFmod enables you to run various synched files, a bit like "tracks" on a multitrack program. Then when your player enters danger, FMod will fade up the tracks that for fighting music, or when you enter a pretty valley, the strings and harps play... YOu still have to write and record the music, but FMod allows you to cue them.
How it works in Unity is another question, dont know, but some have tried.. Do a search for FMod in Unity and you find a bunch of stuff...
not really PlayMaker related however, but IM sure you can use PM to trigger/fade in tracks in a rudimentary way, you just have to be careful on what the music is.
If its very beat oriented it will def sound odd as you fade from one part to another, but if you use non beat oriented for background (pads and washes of ambient sound) then only bring in drums and percussion on fights or excitement cues, then Im sure you can do some cool stuff.
Design your music around what you can do seamlessly, and no one will react, if you try to copy things you have heard elsewhere, you may get stuck... Remember endlessly repeating music is a nightmare, Its probably fine to bring in a piece on entrance to an new area, have the music play out and fade out to atmospheric sounds. Or bring in some low rumbling drones when entering risky areas(but no attack yet) then BAM BAM BAM on the percussion as the hoardes of Zombies leap out of their hiding places...
MArk
(professional film music composer)