If you just want to raise or lower the volume, you should change the M0 to M1 (M1 tells the modem to keep the sound on until the modem makes a connection) and then use the L command with it to tell the modem exactly how loud or soft you want it's volume to be. Be sure there is no other M__ command in the string (no M0, M2, OR M3) nor any other L__ command other than the one you want to use. The L value is what controls the volume level. The command choices are (L3 = high), (L2 = medium), (L1 = low), or (L0 = lowest, with the 0 being a zero).
To turn off your modem's speaker, put M0L0 (0 is zero) in your dialing program's modem initialization string. If you are using Dial-Up Networking, enter the command in Extra Settings, which is located under Control Panel > Modems (or Phone and Modem Options - Modems) > Properties > Connection > Advanced.