@dervheid / @Cheap Converter / @Richard Sload
The Nokia N91 (released quite a while ago - I must have had one for a couple of years, perhaps a few months more) was I believe Nokia's first phone to have a built in 3.5mm jack. This means you can plug headphones straight in. This has worked for me with Sennheiser headphones at home, Sony and Sennheiser earphones when out and about, computer speakers in the kitchen, various 3.5mm to 2xphono converters for connecting to HiFi at home and mixing desks at events and a couple of different 3.5mm to tape converters in the car (basically, anything standard works).
If there is a call the in-built microphone still works, so you can take calls - this is particularly useful in the car with the phone mounted on an air vent, as it means that the car's sound system acts as a hands free kit without having to worry about connecting the inline remote/mic that comes with the phone. If you're walking about then you would need to take the phone out of your pocket to answer it and talk (as the microphone in use is the one in the phone), but can leave the earphones attached and the sound will come out through your earphones.
The inline remote/mic has a four-pole 3.5mm plug that goes into the same socket that you can directly plug headphones into, and another connector built into the plug that goes into an adjacent connector on the phone (later N series phones, such as the N95, N81, N82 & this N78 have managed to dispense with this extra connector). I assume the 4th pole is for the mic (just connected to earth when you plug earphones directly in), and the other connector for controls (with the newer phones piggy-backing the two together on the fourth pole somehow). The remote/mic then has a 3.5mm socket that can fit all the above mentioned devices.
With the inline remote/mic in place, you can control the music (play/pause, previous, next and stop buttons), and answer/end calls and make calls and switch profiles (the call button pressed on receiving a call answers it, pressing during a call hangs up, and while not in a call holding the button down starts the voice recognition up, which allows calling your contacts and changing the profile (eg. you say silent or general)) and adjust the volume (works in both calls and the music player), all while leaving the phone in your pocket. There is also a separate keypad lock switch, so you can have the phone locked and remote unlocked, both locked, both unlocked or (and I don't know why you'd do this, but you can) the phone keypad unlocked and the remote keypad locked. It will use the mic in the remote when it is connected, rather than the one in the phone. This can also be useful for some video applications (put the phone on its charge stand, point at a musician in a set location - eg. a pianist - and put the remote mic in the optimal position for the sound recording and hit record).
I don't tend to use the remote a lot these days (although I have used it a fair bit in the past), as the N91 has big music buttons that can be operated in a pocket, and I don't mind taking the phone out to answer and as I said above, I don't need it in the car, but on these phones with more crowded music button areas it might be good.
I really appreciate the in built FM transmitter feature on the N78, as many cars nowadays have radio and CD but no bluetooth A2DP, tape or easily accessible line in (although I am lucky enough to have a tape still, allowing me to easily plug in my N91 through a cheap easy to use widely available and normally decent sounding adapter), so kudos to Nokia for that. I hope the N96 has it too (I have heard mixed reports on that - the initial announcement didn't mention it, but not checked the official specs recently).
Anyway, hope that rather long message has clarified the whole nokia & headphones thing.