As Steve mentions, there's no need to change things from stereo to mono. If you want to adjust the volume, edit the songs gain/dB levels. Keep in mind that by raising it too high, you may just distort the sound quality. This setting can be found on the left side of the screen, it's the -/+ scale. Leaving the bitrate at 320 is probably the best idea and you can just leave the equalization/compression =).
You can easily use any mp3 audio and make multi-compatible ringtones by reducing the sample rate to 22k Hz instead of 44k. You will not lose appreciable quality and you will insure that any ringtone that you send to a friend's phone will be compatible. Also, to quickly increase volume you can highlight your audio selection and then go to Effects>Amplify. The plugin will automatically amplify to a non-clipped level, but you can also overide this by putting a checkmark in the "Allow clipping" box if necessary.