If that's the folder you've extracted unisound.com into, then it won't be in the PATH variable, which is where DOS checks for programmes that you type at the command line.
Either put the FULL path in front of unisound.com in autoexec.bat, or, preferably, edit the PATH variable in autoexec.bat to add the unisound folder, so that you can run unisound.com anywhere you are.
Something like:
PATH=C:\DOS
PATH=%PATH%;C:\WIN98G~1\DRIVERS\YAMAHA~1\UNISOU~1\
unisound.com
or
PATH=C:\DOS
C:\WIN98G~1\DRIVERS\YAMAHA~1\UNISOU~1\unisound.com
The former is the better long-term option, as you are likely to want to call unisound.com before/after a game to set volume options... and you don't want to be typing that nasty long filename path over and over again.
My collection database and technical wiki:
https://www.target-earth.net