You can use a HDMI audio extractor ahead of your projector to send it to the amplifier first. This will give you the best possible audio levels available.
That's only about 20 bucks and can give the best results with your setup.
The obvious issue being that you are using very bottom rung gear to get things working at all.
It might be worth perusing your local Craigslist, or even hit up eBay, to look for an older non-HDMI AV receiver that has better power delivery if you want to get a lot more power out of the system as well as support for digital optical audio connectivity, which that HDMI audio extractor will give you. It won't force you to buy a brand new, and potentially expensive HDMI equipped AV receiver right now, but can deliver 100+ watts per channel to drive your speakers properly.
For example, in my area...
Yamaha AV receive in good condition.
washingtondc.craigslist.org
That is $15 well spent if you just want to get some decent amplified audio to some speakers using digital optical audio.