It has nothing to do with the screenprotector.
I have both devices side by side and called to my voicemail on the same number and the phone speaker on the Raphael is definitely not as strong as the one on the Diamond.
I have a hard time deciding which device to keep as speaker volume is important to me.
But the volume on the Diamond speaker is much higher than on my previous devices as the Touch, TyTN II, etc.
Hope HTC issues a ROM update to increase the volume as the hardware should be identical
HP Jornada 680--> Casiopeia E 105--> Ipaq 3630--> Ipaq 3970--> Toshiba E800--> Qtek 2020--> Imate Jam--> Ipaq HX4700--> Imate Jasjar (WM6)--> Imate KJAm--> Imate Jamin--> HTC P3600-->HTC P4350--> ETEN X500+--> HTC TyTN II--> HTC touch plus--> HTC Touch Pro -->HTC Touch Diamond + HTC X7500 + Iphone 16 GB White + HTC Touch HD