Hmmmm.. So long as you didn't actually change any of the parameters in the service menu you should be OK. Pretty sure the way those change is by keying the mic briefly while the parameter is displayed, when in the service menu.
So, as long as you didn't key up while in the service menu you should be fine.
I strongly suspect the speaker is the problem because you said an external speaker works fine, it's only the internal one which is low in volume. Please correct me if I got that wrong !
The internal and external speakers are selected by a switch inside the ext speaker plug so they share the same circuits. If one is working, the other should also work, unless of course it is blown/defective.