Meh. Unity 5 just got released today.
I've reinvented the wheel, I swear.
There wasn't much information in the Internet about seamless loop in Unity. Even less with looping from a certain point of the bgm.
I mean, no information.
So, here's a script I did that solved all of my problems.
Here (Note the MIT License)
To use it you need to do, somewhere in your code, something like this:
And it should work.
Btw, referent to a certain comment, if you want to keep a bgm playing between levels...
Type this in the Play function, after the AudioClip.Create:
Don't worry, that temporal Audio Clip is destroyed on each ChangeTrack call.
Again, it should work.]
[I did write some detailed explanation here, but somehow it got deleted. Heck]
A low-count-word (?) explanation:
I retrieve the full contents of the AudioClip, then I play it and when the bgm loops I wrap to the loop position.
Because the AudioSource time sets the time on every loop to 0 and not MusicLoopPoint, I made the Time and SampleTime properties that will return (and that you can set) the correct current time.
Note that it doesn't have Stop, Pause or similar functions. I didn't need them and I though that could be an exercise for the reader.
[The culprit of not having a detailed explanation is a Show/Hide button that I tried to insert here!]
To use it, attach a AudioSource to any GameObject, uncheck Loop and Play On Awake, attach this to some GameObject (can be the same) and drag n' drop the AudioSource to the Audio Source property in the Inspector. You can set the Spatial Blend to whatever you like.
Set the Audio Clip property to some awesome music, and specify correctly the Music Loop Point in seconds.
The Audio Clip must be decompressed on Load and I recommend Preload Audio Data and PCM compression.
Here's the code if the download fails:
Never, ever, trust Show/Hide buttons...