There are three English language radio stations in Korea that I know about: AFN (AM and FM) and TBS-EFM. There may be more, but these are the ones I've found.
AFN is American Forces Network. They operate television stations as well, but those are only available to US SOFA status personnel with a decoder. The radio can be received by anyone. From the website:
We only have one FM and one AM channel (unless you lease DTH). We attempt to program for the majority of our audience. Most of the music broadcast features a mix of current hits, classic rock, Top 40, urban and rock music. Our AM channel features news, sports, talk, oldies, and country music.
Location AM FM
Chinhae (Chinhae Naval Base) 1512 88.5
Daegu (USAG Walker) 1080 88.5
Dongducheon (USAG Casey) 1197 88.3
Gunsan (Kunsan Air Base) 1440 88.5
Gwangju (Gwangju Air Base) None 88.5
Pohang (Camp Mujuk) 1512 None
Pyongtaek (USAG Humphrey) 1440 88.3
Seoul (USAG Yongsan) 1530 102.7
Songtan (Osan Air Base) 1359 88.5
Uijongbu (USAG Red Cloud) 1161 88.5
Waegwan (USAG Carroll) 1080 88.5
Wonju (USAG Long) 1440 88.3
TBS EFM 101.3 This radio station is fairly new and plays lots of requests from both Western music and Korean music. It has lots of great information for the expat. Segments such as how the buses work to current festivals. Today, they talked about a dating services that targets expats. Sarah and Sejin did a quiz on this radio station. So check them out.