Silencing the Priest? Priesting the Silence?

Since the introduction of the OF Mass, the priest's prayers have been recited aloud, unlike the custom of the EF.  GIRM #32 is very clear: 

The nature of the “presidential” parts requires that they be spoken in a loud and clear voice and that everyone listen to them attentively. Therefore, while the Priest is pronouncing them, there should be no other prayers or singing, and the organ or other musical instruments should be silent.

OK, then.  Why do we sing an "offertory hymn" right over the 'presidential prayer' of the priest?  That is the practice I have observed in two different parishes, both claiming "orthopraxy".

Egads.