Chosen Solution
Why can’t I hear anything without my headphones in? It’s like the phone thinks the headphones are in.
This is a belated answer so hopefully it can be of use to folks who search for and find this question. I would suspect that the headphone port is damaged or gunked up, causing the pins that contact the headphone jack to remain depressed. The phones interprets this as a headphone being plugged in. You could take a look in the port with a bright light and a jewellers loupe to see if the is the case. If so, then try to wet a small cotton swab with some IPA and try to clean out the port. In a worst case scenario, a replacement port should solve the problem.
