Grumpy_Mike:
I cannot connect the buzzer to a higher voltage
Then you can not put more current through it.
You could generate a higher voltage with an inverter or transformer.The fact that it sounded less loud through a transistor suggests you did not wire it up right. Emitter to ground, buzzer to collector an +5V?
Mount the sounder on a ridged surface to make it sound louder, the inside of the box would do.
Ok I'll try to fix it better. On the other side I understand that the only possibility is to increase voltage. I'll think about it.
Thanks for your comments!