There can be several factors for "flat" sounding audio. The original recording, your car stereo's settings, the transmitter, the volume of the audio going into the transmitter - just to name a few. Although in this case, I doubt it's the transmitter. The EDM transmitters have filtering which definitely "clean" up the sound from the Pi. We did a side by side transmitter test at the last Denver Mini. My FM02 picks up some interference and the sound is "warbled" from the Pi. PK (my dad) is working on a solution, but I digress.
EQ'ing may help. Compression does help. Filtering the audio to the frequency response of the transmitter will help.
If I may ask, what were you listening to?