Quote from Hardmuscle
It will work not might work, if you make it where 1 copy is sound and 4 copies are blank then you're making it play nothing 4/5ths of the time, it will only play the sound file one time and choose random from variations.
(if not on a pulser obviously)
For example you have /playsound mob.ghast.scream @p in a command block set to play one time via a proximity detector/clock (testfor clock), when the sound begins it will play 1 of the 5 variations of the file named ghast.scream randomly (ghast.scream1, ghast.scream2 and so on), it will not play them all at once. So if all the variations of a sound file are the same, you will get the same resulting sound 100% of the time instead of 1/5th of the time.
Having blanks will play blanks and blanks are blank and boring lol and we cannot currently choose specific variations at this time (1,2,3 etc etc), ..with commands, though you could make your blanks 1 second long lol but that'd just be darn right laziness lol.. sigh
I can see that you didn't try it yourself use a different rain.ogg file that is longer in time than the default rain drop sounds.
Because all of your theory about how it should work is worthless in the case of how the rain is coded in game.