You are preloading the files correctly, so what's left to check is: isn't there a silent gap in the mp3 file? You'll want to open it with a wave editor and check it.
If after doing that you're certain the wave starts at the very beginning, then maybe converting to another format like ogg or wav may help.
Edit: second parameter to _SNDOPEN "LEN","VOL","SYNC", etc, is not required anymore as of version 1.2. They are all set by default anyway and the parameter is simply ignored.
Edit2: If you intend to play a loaded sound several times in a sequence (like repeating a note or a sound effect in a game), use _SNDPLAYCOPY instead of _SNDPLAY.