"Multiplayer" ATC flight simulator x

hello need a little help here. when playing multiplayer I have my headset plug in right. I can here the control tower talking to me but I cant talk to anybody..Hitting the caps button to talk. in the start up menu I was reading in atc multiplayer says I need 2 sound cards? I have a sound blaster 24 and a onbord sound card do I need to enable it too? Also when Talking to ATC and other craft do I need to set the radio freq. the same in order to talk to them.

thanks.

what if i can't even hear anybody or talk to them. im on the right freq. and have the headset plugged in correct and it works.

press shift caps lock