.TP
\fBDOOMWADDIR\fR, \fBDOOMWADPATH\fR
These environment variables provide paths to search for Doom .WAD files when
looking for a game IWAD file or a PWAD file specified with the `\-file' option.
\fBDOOMWADDIR\fR specifies a single path in which to look for WAD files,
while \fBDOOMWWADPATH\fR specifies a colon-separated list of paths to search.
.TP
\fBPCSOUND_DRIVER\fR
When running in PC speaker sound effect mode, this environment variable
specifies a PC speaker driver to use for sound effect playback.  Valid
options are "Linux" for the Linux console mode driver, "BSD" for the
NetBSD/OpenBSD PC speaker driver, and "SDL" for SDL-based emulated PC speaker
playback (using the digital output).
.TP
\fBOPL_DRIVER\fR
When using OPL MIDI playback, this environment variable specifies an
OPL backend driver to use.  Valid options are "SDL" for an SDL-based
software emulated OPL chip, "Linux" for the Linux hardware OPL driver,
and "OpenBSD" for the OpenBSD/NetBSD hardware OPL driver.

Generally speaking, a real hardware OPL chip sounds better than software
emulation; however, modern machines do not often include one. If
present, it may still require extra work to set up and elevated
security privileges to access.