I do not know much about the PC side, but on Macintosh there was
the excellent MidiGraphy editor that runs under OS 9.  Alternatively,
you could get a hex editor and change the events yourself.  (I have
a few tools that must run under Windows, Winhex is one of them.
Alternatively you can use the debugger built into DOS.)

The play tempo is changed one of three ways.  In Standard MIDI Files
Type 1 and Type 0 there is a "meta escape".  This can change the tempo
divisor at any time in the file.  There can be multiple instances of
this.  Microsoft prefers that you use a Type 1 MIDI file and put all
the tempo changes into Track 0.

The next most common way to change the tempo is to re-sequence the
MIDI file with all new delta events between the notes.

The third way is to change the hardware clock of the playback computer.

The playback volume is more easy to change.  There are lots of tools
for that.  Look on sites that cater to solenoid pianos, like the Yamaha
e-group.  I also recommend the tools on Spencer Chase's site
  http://www.spencerserolls.com/

Spencer has also been recommending <vanBasco's Karaoke Player> download at
http://www.freedownloadscenter.com/Multimedia_and_Graphics/MIDI_Players_and_Editors/vanBasco_s_Karaoke_Player.html

I have not personally used the latter, but it looks promising.

Julie Porter