You need to be carefull with FM Transmitters. Use this website Radio Locator to find stations being used in your area. Don't use those on the list. Second is scan for stations on radio. If something tunes in not on the list, don't use that station either. There are FCC regulations on transmitting, you need to self educate on the laws.
	The Antenna that comes on a lot of transmitters are junk and noisy. Use a dipole antenna tuned to the frequency you are broadcasting on.
	A Sound Card with a good DAC (Digital to Audio Convertor) is needed. The Sound Blaster 3 meets needs, and you can find one for roughly $20.00.
You will need a quality sound card. The Soundblaster 3 shown above is compatible with a Raspberry PI running FPP. The dipole antenna is your second most import part of the audio. The transmotter is important, and not broadcasting with to much power is key. If your audio sucks, you lose a big part of the show.