How to differ Ear speaker from other models?

I don’t really understand the question either unless they are talking about soldering the speaker to the flex cable. I would try Siri if she understands it would be the speaker. Also I would try voice memos as a muffled sound could be charge port mic and speakers especially if it’s the other end of the call that is muffled. I wouldn’t change anything out until I knew which part was causing the problem.

The audio of the other person's voice is muffled when he puts his ear to the device.

Like he has to put his phone on speaker and talk that way often because the call audio isn't loud enough

A lot of the time the dust filter on the screen just needs to be cleaned. A minute with a fine bristle brush should do the trick. If not you'll need to clean both sides which will mean removing the earpiece speaker also to clean as well behind the screen where the filter is.