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.
It's a shame that there are some limitations, that, unless some mapmaker or mindcracker speaks to Dinnerbone, will never be changed.
If someone has a twitter account, would be great to tweet him this.
The major limitation is the rain sound.
It is not a single file on repeat... it's actually four separate "rain drop" sounds that are hardcored to be randomly played.
So if you wanted resource packs to play something like "rainy mood" when it starts rain ingame, too bad... the game will only play the first second of the complete audio over and over again.
We cannot escape the annoyingn rain... the only option is to replace the drop sounds with a lower volume copy, or with a mute one.