Why can't I hear my calls through wired external speaker, but I can with wired headphones?

[AC Question]

I just transitioned from stock Kit Kat to Marshmallow on my OnePlus One, and so far, haven't noticed many problems. However, I have an issue that I can't figure out. When I plug my wired headphones into my phone to listen to my music, I can hear my music and any calls or notifications can be heard through the headphones. However, when I plug a wired speaker (tried with aux in car and a small portable speaker) into my phone to listen to my music, I can't hear the call through the speakers unless I place the phone into speakerphone mode. This, of course, causes an echo for the person on the other end of the line. Any idea what's going on and how to fix it? Thanks in advance for any and all help.