The MT5634SMI-34 SocketModem gives an error when I give it the AT+MS=? command from Hyperterminal. Does this mean that it doesn't support V.92 commands? If not, then how do I change its modulation scheme to V.34?

The MT5634SMI-34 firmware builds do not support the V.92 commands, so the +MS command is not supported on the -34 build. This limits the modem to the V.34 / 33.6k bps and slower standards; so you don't need to give it a command to prevent it from going faster than V.34. If you want to prevent the modem from falling back to lower speeds during handshaking, use the command ATN0S37=19&W.