I decided to buy a pair of headphones with remote control. Firstly I bought a really cheap AD-54 adapter, which has play/pause, next, previous, stop, vol up, vol down buttons and 3,5mm input. It is working fine with my N9. But I wanted to give it to my sister, so I bought a new pair of white (it's called "ice blue") WH-701 headphones.
I got them and... they do not work. They work well with all other Nokia phones (for example C7), but they do not work with my N9. I plugged my friend's black WH-701 into N9 and they do work.
When I plug my WH-701, I can hear only two squeaky sounds from headphones and the music comes from the phone's speaker. The buttons and the microphone do not work either.
I wonder if the phone identifies them as the video-out cable, or something?
Is there any way to fix it, or force the phone to play the music trough the headphones?

WH-701 headset should work correctly with N9, including its media control buttons and so on. I have used it myself.

So your new WH-701 is not working with your N9 but your friend's WH-701 does work? Maybe your WH-701 is just somehow faulty? Warranty? The phone does not detect the headset correctly if audio remains in the phone speaker.

I can confirm that WH-701 is working correctly with the N9. So get yours exchanged.

Interestingly I just wanted to create a thread with my own problems with N9 and WH-701. I've observed the following strange things:

On my N9 64GB with plenty of software installed plugging the WH-701 in results in the charging LED to light up and the the device rebooting. This can be reproduced consistently.

On a N9 16GB which is more or less as delivered by Nokia pluging in the WH-701 once led to a reboot, once led to a failure of the plugging-in to be recognised (payer continued to play on speaker) but on several other tries seemed to work normally.

The only software installed that makes use of the extra buttons to my knowledge is FMRadioN9. Does this software run anything in the background?

Any ideas what could be the cause?

Edit: And yes, the headset works fine with phones that support it officially. So it's no hardware issue on the headset part.

I cannot exchange the headphones - they work fine with any other phone. And the phone works fine with other headphones ;/

I've had some time and have done some more testing. Now the WH-701 works as advertised. I've no clue why. The phone has been rebooted since and some software was updated. Take your pick. (Given that plugging in in the headset earlier caused a reboot I wonder if rebooting really changed anything?)

I've then tried a BH-905i (which worked flawlessly earlier over Bluetooth) connected by cable, and it worked fine too. However when connecting it by Bluetooth afterwards it still worked fine except for the N9 not showing the volume bar anymore when changing volume with the BH-905i.

Extra buttong support via cable connected headset seems to be a bit buggy ?

When I connect WH-701 to device sometimes it works and then I see following in dmesg:

[124036.370391] twl5031_aci twl5031_aci: ECI as SND_JACK_HEADSET

But more frequent result is negative and in this case it says

[147594.968536] twl5031_aci twl5031_aci: AV jack open cable as SND_JACK_LINEOUT

Do somebody know how to hack twl5031_aci and force it always recognize WH-701 as SND_JACK_HEADSET, not SND_JACK_LINEOUT? This particular headset always work properly with E7.

Well, I bought another pair of WH-701 and they do work with my N9. Probably the first ones were not original.
@linuxovod: maybe you should plug them with a one, smooth move? The difference between headset and lineout is that headset uses 4 pins, and the lineout - only three, the one closest to the cable is as long as two pins of the headset. So when you plug them not smoothly (I do not know how to describe it) you probably connect those two pins in the device with the headset plug (or maybe something inside your plug is wrong and there are some unwanted connections?).
I know, that you ask for something else, it would be more helpful if we could force it with a terminal, but I do not know the answer for that.

Interesting, I just have some of the problems mentioned. At first my WH-701 didn't work with any phone, not even my N900. Then I discovered they worked fine, when pulled 1mm out, then they worked ONCE with my N9 flawlessly (not the buttons though) and now they don't work with my N9 again, regardless of "Dolby"-Setting and reboots with or without headset plugged in.

edit: interestingly the N900 detects the plugging-out-process and stops the music, even though it doesn't play through the headphones.

Have been searching for a good headset to use with my N9.
I wanted premium audio combined with remote/mic functions.

Well, in the end I must say that this combination doesn't exist. The best fit here would be the BH-905i, this one works good but from ergonomics point of view this isn't my preferred choice (still looking too bulky)

So what's the solution for me? I bought a pair of B&W P5 and use them together with Nokia AD-54. The AD-54 is an essential part of the whole experience as you can use the multimedia controls as well as the built-in microphone.