BaoFeng UV-5R won't hear aprs with headphones?

[moemoemoe]

I'm using a BaoFeng UV-5R2+ to pick up APRS packets. When I tune the radio to the APRS freq (144.39 MHz) I can hear them coming across perfectly on the radio's speaker

However, if I plug a 2.5mm to 3.5mm adapter into the speaker jack and then I plug my regular iphone headphones into the adapter then I can't hear any packets at all.
If I use this exact setup on another freq (weather freq for instance) I can hear everything just fine. It only seems to be an issue with APRS packets.

Any ideas on what the issue could be?

Note: the end goal here would be to plug the radio into a computer (radio speaker out to computer mic in) to decode the packets.

 

[jonwienke]

Using a stereo 2.5-3.5mm adapter, you're only going to hear audio in one of the two speakers/earbuds. Are you listening to both, or just one?

 

[moemoemoe]

I can only hear the tones through a single earbud. I can only listen through one earbud.