Recording video w/ audio thru a mixer

I'm trying to record a video on my iPhone, with good quality sound. I'm singing to accompaniment music playing on my MacBook Pro. Using a Yamaha sound mixer, a studio mike, and connecting it from the iPhone lightning port using a Rode SC4 adapter, and running it into the RCA outputs on the mixer. The sound comes through, but both vocals and accompaniment overmodulate and distort, even on low volume settings. Headphone volume is turned all the way up, but the accompaniment is still barely hearable through the headphones. What am I missing? --Tom in Texas