how do I shrink the file size for the music files for mh3u??

I want to change the music so I can hear fireball when fighting the los/ian family and turn down for what when I complete a quest... is it possible??? it runs mca files but the folder says wav... when I save them to mca in audacity, it makes them 32,000+ kbs... is there a way to shrink them??? thank you in advance