NoMidi Variable:

If an environment variable named NOMIDI is defined when the plugin is loaded, MIDI will be disabled and APING will not make any sounds. (The contents of the variable don’t matter, so long as it contains something.) Setting this variable allows you to use the plugin on systems which don’t support MIDI:

set nomidi=1
plugin /l c:\bin\tcmd\test\aping.dll

Setting the NoMidi registry variable to a nonzero value does the same thing.