Ive been battling this problem for some time, whilse teamspeak and steam could properly detect my microphone and its transmissions, ARMA would not transmit any sound, but i have now found a working solution
Right click your sound icon in the lower right part of the screen, and click "Recording Devices".
check your default device, if it is anything other than your microphone (for example, my sound card was selected as the default device), then right click your microphone and select "Set default device"
this shouldn't even need an ARMA restart, it seems ARMA uses your default recording device for VOIP rather than your default communications device
hope this helps some people :)
