I work in radio here in the states, so I did take some time to check out the dial in Ambergris Caye during my visit last month. You have 2 stations on the FM band:
92.3 Reef Radio (or Radio Arrecife in Spanish)
The only true taste of local radio, you will hear some local music, along with whatever is popular in the States, from hip-hop to country - it just depends on which DJ is on the air. That also determines whether what you hear is in English or Spanish. For true local flavor, the morning show is worth checking out - it's mostly in English and is akin to what you might hear on a small-town AM station here in the states.
There is a religious/contemporary Christian relay station on the island as well (I believe 101.3?)
Belize City's "LOVE FM" is listed as having a relay at 98.1 in San Pedro, but it was not on the air when I was there last month. (You can, however, hear it in Belize City airport as most of the stores there have it on - and it does give a good sense of the "Belize sound")
There is no local AM reception - the only AM's you'll catch during the day are Mexican: 960/XEROO "La Voz de Quintana Roo" (Chetumal) and 1050/XEQOO "Radio Pirata" (Cancun)
Of course, at night you'll have a wide-open band with various US, Mexican and Cuban high-powers coming in via skywave.
Honestly, the only time I "really" listened to the radio while in San Pedro was late one night when I was able to catch WIOD-Miami with their hurricane coverage - the rest of the time I was far too distracted to even think about listening to the radio! I think you'll find the same thing will hold true during your visit!