Yes you can put a boat cover in a washing machine. I did my 2008 Malibu VLX cover (non sunbrella) on hand wash setting in my LG front load washer last spring. Put it on the boat to dry, then sprayed it down with waterproofing spray (cover off the boat). The waterproofing is already worn off though and i need to do another coat for this season.
Depending on the severity of the tear in the cover, I would price out an upholstery shop to stitch up the torn cover, wash it up, sell it on craigslist and buy speaker sox and a stand alone platform cover. I just dont see how you could piece together boat cover material to make speaker covers. Platform would be doable, but would require someone with sewing machine experience or $ to have a shop do it.