GRUB_INIT_TUNE uses the following format:
tempo freq duration [freq duration freq duration...]
tempois the base for all note durations in beats per minute (60 gives a 1-second base, 120 gives a half-second base, etc.)
freqis the frequency (in hertz) of the sound (set 0 to produce a rest)
durationis the duration of the sound in a 4/4 bar (meaning 1 is a quarter of a bar and 4 a full bar)
That's how GRUB is designed, so bear in mind while tune is playing, booting will be paused. Try not to use long tunes.
Tunes will be played on the PC speaker at full volume. There's no way to control volume. If your speaker is too loud, a short tune is recommended.