Hi Malik! Yes, when speaking some sounds can change just like in English (“I want to” → “I wanna”, etc.), but here that is not what is happening.
It is not that wa is becoming ya. It is that that they are using the verb k’à faamuya instead of k’à faamu.
Obviously, the two verbs are related — -ya is a sort of abstraction suffix for verbs — but they are they distinct.