Adjust TX and RX gain (AsteriskNOW)


I need help on adjusting TX and RX gain through my Analog Telephony card on my PBX server. the sound seems very low when i call through the dahdi channels.

I have the TDM808EF (Analog card) with 8 FXO modules.

I have AsteriskNOW on an CentOS Linux Operating System, with FreePBX and Asterisk .

Thank you for any assistance.