So I had this problem, but at first when I tried to fix it I just entered in the command into the GOG shortcut. If you're reading this and that's what you're doing: you're doing it wrong. As OP stated MAKE A NEW SHORTCUT directly to the BattlefrontII.exe, the GOG shortcut actually takes it through another launcher all together that doesn't know what to do with the audiomixbuffer command.
If you make the shortcut to BattlefrontII.exe and add the command to it, you shouldn't have any problems (though you may have to make the mix buffer larger depending on your machine, apparently up to 200 is fine). Figured I'd comment this as I'm sure I'm not going to be the only person who tries to outsmart the instructions at first.
Thanks for the fix OP.
Post edited October 10, 2017 by AnotherAvatar