To get the best quality, record with wired headphones! Often times on iOS, the older, wired earpods work best. Any wired headphones on Android will do too.
Bluetooth can be a struggle and sometimes will cause the vocals and beat to be off (sorry fam but nothing we ourselves can do here).
Also, after recording you have the ability to update volume controls for vocals, and beats… so if the beat is too loud and your vocals are too soft, use the mixer tool to adjust them.