The following instructions describe how to install Notation Composer and run TiMidity++ (hereafter referred to as Timidity) as an ALSA sequencer.
Open up a terminal.
The installation program for Composer requires mfc42.dll, which can be obtained with winetricks:
sh winetricks vcrun6
Run the installation program for Composer. Here, I assume it is in the Desktop directory, and that the version is 2.3.3.
Composer version 2.3.3 will start automatically after installation. Please close it to continue.
MIDI playback requires the presence of a MIDI sequencer. Timidity works well for this. If Timidity is not installed on your system, check your distro's repositories. You will also need an instrument patch set, such as Freepats. Slackware users can find the necessary slackbuilds on slackbuilds.org. When Timidity is up and running, start it as follows (I urge you to read the timidity manpage to understand each option):
timidity -iA -B2,8 -Os -EFreverb=0 &
cd ~/.wine/drive_c/Program\ Files/Notation/Composer
If all went well up to this point, open up a midi file, click the play button, and enjoy! If you do not hear sound, make sure your speakers are on and check your volume levels. Then, click "Setup > MIDI devices quick setup..." and verify that "TiMidity – TiMidity port 0" is visible under "Select MIDI Playback Device." If it is not, make sure you followed step 5 correctly. Next, click "Test Playback" to listen to a chromatic scale. If you still do not hear anything, Timidity may not be configured properly. Check your distro's documentation and/or forums for any hints on installing and configuring Timidity.
Note: ALSA Playback
On my machine, in Slackware 12.0, the snd_seq_dummy module is loaded by default. When using ALSA in Wine, this module seems to conflict with the actual module for my sound device, snd_hda_intel, causing frequent crashes, no audio output, and an almost unresponsive interface when attempting playback. The solution, of course, is to remove that module (modprobe -r snd_seq_dummy) and blacklist it in /etc/modprobe.d/blacklist. The location of the blacklist file varies between distros.