There are two settings for Disabling/Muting Voice chat. One is just a simple mute (for yourself), you can find it somewhere in the menu that's also used to place the party beacon/etc. The second option turns off your ability to hear others and them to be able to hear you (You obviously don't have this setting on, since you can hear them.) The second option is located in settings under chat settings. (Not chat settings in the context/flyout menu on the gears/cogs, but options/settings and then chat settings category.)
I have the second option on, as I don't want to be able to hear randoms/etc. that come into my parties. My UK friend also has turned that setting on too. And yes, you heard me right, random people who join your party can voice chat with you (as long as they join your party.) If you don't have the voice chat disabled setting on.
Also, if you are chat banned, you can't speak either. (If you can text chat publicly, you aren't chat banned.)