1) Regarding video connection, you can simply buy a DVI-I to VGA converter dongle, DVI-I to DVD-D cable, DVD-I to DVD-A cable, DisplayPort to DVD-D cable or DisplayPort to HDMI cable, or DisplayPort to DisplayPort cable to plug into any display with a VGA, DVD-D, DVD-A, HDMI, or DisplayPort connector (make sure you get the right gender though). You can pick these up on Amazon or other stores for about $7 to $35.

2) The Chromebox jack is intended for earphones and headphones rather than external speakers, and the jacks impedance is not correct for speaker jacks. I don't use an amplified speaker, but I believe a ground loop filter will solve the problem - http://www.ebay.com/itm/MINI-3-5MM-NOISE-FILTER-GROUND-LOOP-ISOLATOR-CAR-AUDIO-/260741338139#ht_3540wt_1141

3) If you want to allow guest accounts to use, then on a non-enterprise Chromebox, you just log in as the main user or administrative user, select settings and help -> Wifi, and check the "share this network with other users" checkbox, and guest users will use the WiFi without having to supply WiFi security key credentials.