Here's something that has an Aux Mini input, is widely available used (ie: eBay), and should prove much better than the speakers in the projector (even when they were new):
https://www.amazon.com/Ion-Sport-High-Power-All-Weather-Rechargeable/dp/B09T5445SK
on eBay:
https://www.ebay.com/p/9034205085?iid=374206575133#ProductDetails
If one doesn't cut it: buy a couple and a 'Y' mini connector.