MightyAfter some 'experimenting', I think that the reason why it switches sounds for the original is because there are multiple firing sounds for it.
If you transfer any sounds to the normal rocket launcher, they work fine, since it only has 1 firing sound.
I've not done anything with this, but are there not multiple lines of text distinguishing the different sounds? I just know that w/ the explosions there were different effects for directs and things like that