MIDI is digital data only and must be "assembled" by a MID Synth, it's not analog audio. MIDI playback is generated by a MIDI synthesizer/player and either sent to the PC's sound card or to some external sound device. Any "volume control" would be on the MIDI device or program. I use Sonar and the MIDI synths in Sonar are what drives the MIDI sounds and I control the volume level in Sonar.
I also have a Focusrite Saffire Pro 40 unit.
When I use Band in a Box, which uses MIDI, it "assembles" the MIDI digital data and sends it to the PC"s sound card where it is processed like any other digital audio format such as a wav, MP3, etc and then the sound card amplifies it.