It's unlikely the manufacturer could do anything. They design products with little hardware customizing in mind. If it were possible to make it louder, then you'll risk damaging the speaker (running an amplifier at maximum increases distortion, and distortion is what kills speakers quicker than anything).
You can go check the accessibility settings in your phone's main settings and find hearing related options. One of them spoils flash the LED during incoming calls. A smart watch would work, but those are expensive. A Bluetooth speaker would be cheaper, but louder and play those notifications. In either of those cases, you're also going to be limited by how far away you get from each other. Bluetooth doesn't have great range.