Place the following commands into Notepad (or equivalent) with each being on a seperate line and save as demoloop.cfg in your \Quake3\Baseq3\ directory:
disconnect
s_initsound 0
snd_restart
disconnect
timedemo 1
Set D1 "demo demo127; set nextdemo vstr D2"
Set D2 "demo demo127; set nextdemo vstr D1"
vstr D1
Then at the Q3 console, type exec demoloop.cfg. The demos will loop until you press ESC.
Note that the demos are in timedemo mode and there is no sound. Remove the second and third lines to play in sound and remove timedemo 1 if you want the demos to play at normal speed. Also, this config only works with version 1.27 and greater. If you have an earlier version then you will need to change demo127 above to the name of the earlier demo (I think it is demo001 and demo002).
Hope this helps. . .