Keep in mind that .mp3s are directly loaded into the game and if you use them, they play instantly. It's a different story with .oggs. They are decoded as soon as you try to play them. This will cause bigger delays on .ogg files if they are bigger.
If your mp3s are delayed, get any wave-editor, open them and look if there is some "silence" before the actual soundwaves. Delete that silence and just export them again. I also suggest using .oggs, but as I said, don't make them too big. It's always a good idea to cut them so their playtime is below 1.5 minutes, so the delay while loading them won't nearly be noticable.