try using .ogg files instead ;)
or if you can... host the audio files on a private website/server... and once the game starts load the assets from there... careful the file link on server must be like this depending on construct plugin you using: www.randomwebsite.tld/filesource/music.mp3 if the file format is not present in the link audio won't import.
while this won't make your game smaller... since you will download eventually the full size... what this will do... is have your initial game small. so people can download it, and install the launcher fast. then they will wait in the loading screen of your game while the assets are downloaded, as in any major/normal game you will do. but this method helps you if you want your game to be like just the code no assets to be placed on facebook instant games for example where the requirements maximum files size of game is like 800kb... but that isn't for the whole game, that is for the "snapinest" of initial load only.
if you can't figure the ogg or the server hosting part out, use audacity, and convert your mp3 from 43000Mhz to 16000Mhz mono/stereo single channel, you will lose some quality... like in any compression, but not that big difference for real... unless your device is like full stereo home cinema blueray etc etc etc where sound is a professional thing.