If you’re gonna rock a phone that supports MP3 playback you might as well go for the whole kit and caboodle. The MMR-60 FM Transmitter is compatible with the W810, W600, Z520A, and the W800. Just attach the bottom feeder, tune to the set frequency on the chosen radio and your streaming MP3s. The phone powers the transmitter, so no batteries needed. Available here for $55.
Product page here.