I have the same issue wiht my Huawei E8336D (same hardware, kernel, chipset and all, just an other body..)
when i set a ringtone of my choice (non standard), the volume is remarcably lower compared to the standard one..
I've even converted the .MP3 file into an .ogg file and moved it to the same map as the standard ones, and selcted the ringtone like this.. (I rooted the phone)
still, volume is lower compared to standard ringtones..
next move is to try a custom rom... If it solves the problem for me I'll let you know..