Are you sure that it is through the loudspeakers? I think it is because the sound has been turned up to max. Try during a call to switch to the loudspeakers.
Yes, I think so. If I turn on/off hands free speaker, I would say that it’s the same, I don’t have the phone with me at this moment to test again.
EDIT: I can confirm that the sound is always through loudspeakers instead ear speakers in voice calls, it had nothing to do with volume level is max or not.
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.