Try looking at this >
http://forum.sa-mp.com/showthread.php?t=563907
Go to settings, audio and maximize "RADIO" if it's low, the radio URL could also be broken, so ask someone else if they can hear it before you assume It's client sided.
Delete GTA-SA.set from Documents > GTA San Andreas Userfiles
Ensure stereo mix is disabled.
Go to the 'my documents' folder on your computer, then to GTA San Andreas User Files -> SAMP -> now open sa-mp.cfg with a text editor. Make sure audioproxyoff is set to 1. (audioproxyoff=1)
/togaudiostreams
Last thing I could say is, try reinstalling SAMP.
If NONE of the above work, then It's your firewall, get back to me if any of these fixes it for you or not as it could help others! Cheers.