Thanks for your bug report. If you are referring to the delay inbetween played sounds, there is little mIRC can do about this. The on WAVEEND event is triggered the moment the Windows API notifies mIRC that the sound has finished playing. So before playing the next sound, mIRC has to close a sound, trigger remote events, and play the next sound in the queue. All of this happens immediately in a sequence with no delays, so it is as fast as it can get.
I just tested using seek to reset the sound to the start in the actual code, the moment the API notifies that it has ended, and there is still a break inbetween both sounds. So my guess is that seamlessly playing a sound in a loop would require a very different implementation that is beyond the current /splay.