As long as you use the hot glue sticks mint specifically for high temperature it will be fine. I've done many car projects over the years that have survived this summer's just fine as long as it's the high temp version.what type of glue is it? is it the Hot Glue Gun? if so this will melt in hot weather and your speaker will fall off loose. hope its some other glue that dries..if so please do share with us the type.
also if you can share what brand speakers you used and from where?
I'm more curious if he is the bass sound system or the ultra high fidelity sound. If it's the ladder it will have more power and changing out the speaker should yield more of a result. If it's the former they will be underpowered no matter what so any perceived improvement in audio quality is likely placebo effect.