I don't know if anyone else here is old to enough to remember the original Sony Walkman. I'm talking about the very first model - the TPS-L2 (http://www.walkmancentral.com/products/tps-l2). Well, if you look at the picture, it had a prominent yellow temporary push-button on the unit. This button activated a microphone that would pick up ambient sound, partially mute the sound coming from the cassette player and mix it before sending it through to the headphone output. This made it VERY convenient to be able to hear someone without having to take off your headphones. I have always thought this feature was way way ahead of its time. I'd love to have such a feature with my IEMs as it takes me a bit of effort to get a good fit. Therefore, when I'm at work and need to have a quick chat, I'd like to be able to leave the IEM's in, push a button - have a chat, and go back to listening/working.
Has anyone seen such an add-on or built-in feature on modern hardware?