This is right out of the manual:
SOLO switch and yellow LED: With solo enabled the channel signal is sent to the after-fade listen (AFL) stereo and pre-fade listen (PFL) mono buses. The left and right local monitor outputs can be used, for example, when operating from within a booth to hear selected solos and not the whole mix. The solo LED illuminates to show when a solo is on.
My guess is that you have your speakers plugged into the local main outs, make sure you have them plugged into the master outs. The red fader will control these. Hope this helps. PS: you also will need the solo activated to hear via the headphones. This is to help live sound engineers listen to a specific channel without interrupting the entire live mix.
Actually use the local outs and keep the master fader solo'd at all times. Works better this way, soloing and muting of channels will work as expected, mono mix can be monitored too (just solo it with stereo to mono switch on and stereo master muted). Also the local and phones mutes work as expected, very handy to switch off speakers when cans are on.
I should add, this is the way I use it in the studio. YMMV of course.