Can confirm that this also happens on my 10 IV. Audio always comes from the speaker (at the bottom of the phone) never the earpiece (at the top of the phone). Switching to and from speakerphone while the call is in progress makes no difference. Same result for outgoing and incoming calls on both VoLTE and GSM.
4.6.0.15, newly flashed and nothing else installed. Xperia 10 IV XQ-CC54.