Here are the correct lines to use at the Q2 console:
s_initsound 0
snd_restart 1
timedemo 1
map demo1.dm2
The first turn the sound off for benchmarking purposes. If you want sound, just remove these two, and run timedemo 1 and map demo1.dm2. Just remember when you are finished, if you want to play right after the demo, set timedemo 0 so everything will not be in turbo speed.
I would just cut and paste the above lines into notepad and save it as demo.cfg in your \Quake2\baseq2\ directory. Then you can just type 'exec demo.cfg' at the console.
Save the following lines as sound.cfg in the same directory and type 'exec sound.cfg' to get sound back and turn the timedemo function off.
s_initsound 1
snd_restart 1
timedemo 0
Hope this helps. . . .