Preparing the track
Convert the track to ogg: it weighs 8-10 times less than wav and, unlike mp3, adds no silence to the start of the file. Apart from ogg, only mp3 and wav load
Formats
oga, mp2, aiff, flac and the tracker modules mod, xm, it, s3m are not accepted
flac does not load at all: the engine does not decode it. Convert it first
An mp3 encoder adds a few milliseconds of silence to the start, and different encoders add different amounts. If the track is re-encoded by another encoder, the whole beat map shifts
Offset
Even in ogg a track almost never starts exactly on the first beat. That is what the beat segment's offset is for.
It is set in fractional frames, so it is finer than one frame
The order is: tempo first, then the offset, and only then content
Loudness
Normalise the track before you put it into the level. Peak normalisation to -1 dB is a sensible default
A level noticeably quieter than the next one looks like the game is broken, not the track
Length
The level's length and the track's length are different numbers, they do not have to match. A level can end before the track does. That is how you can make an ending with a fade-out
Several tracks
There can be several tracks. Each has its own position on the timeline, its own speed, volume and chain of effects
Effects: chorus, compressor, distortion, echo, flange, filters, normalisation, equaliser, pitch shifter, reverb
An effect on a track is heard everywhere that track plays, not only where you meant it