Just be careful about creating a ground loop condition from doing this, unless all grounds in your house are terminated to a single point you are introducing the chance for a difference in ground.
Also you can get transient voltage from Cat5 or Coax cables going straight into your PC, cable or satellite set top boxes. If you surge protect everything but not those inputs its just another place for the voltage to seek ground.
I got my Shimian 2 days after ordering from ta_planet for $328 and couldn't be happier, only problem with my unit is 1 dust particle in the lower left quadrant but its hardly noticeable, for the price I am ecstatic!
Why not run HDMI straight to the projector and a separate audio feed? There are devices out there that will strip analog audio from HDMI and then pass the HDMI on to the display.
There are quite a few options these days, the monoprice units are actually very decent. I tend to use a few different brands but have had luck with most of them as long as good termination is applied to the wire.
SirMaster makes an excellent point, speakers have not changed much in form or function over the years and are a sound investment that can last a long time.
I actually have 2 pairs of speakers that are older than a lot of users on this forum including myself and they were more than $150 each...
There are good sound bar units out there, but they don't cost anywhere near as low as the $300 limit. Leon speakers make some great sound bars as well as other speakers but they are not cheap as the tweaters and midrange drivers they use cost more than $300 wholesale in some of the units.