I notice that a lot of replacement cables are terminated with a two-pin plug (or something like that). Could I buy a jack for this plug, and solder it a pair of IEMs?
What is this part called?
I think you are talking about Sennheiser plugs.
There may be 1-2 sources for plugs, but this is the only one I can find at the moment: http://www.cardas.com/content.php?area=oem&pagestring2=Headphone%20Connectors&pagestring=Headphone%20Connectors+%28closeup%29&content_id=33&part_id=178.
I don't know of any sources for jacks, although I have seen it recommended to salvage them from broken Sennheisers that feature a detachable cable.
Don't think the senns are small enough for IEM's though.
Something I've been interested in for a bit of time myself. I use to own a pair of super.fi 5 pros (a couple over a few years). The last pair I tried fixing myself, and negating the cosmetic problem, they functioned again. Hard work, especially given I didn't have the knowledge nor the tools at the time to properly open the shells, solder, seal the shells again, and be happy. Eventually my last pair died, I didn't have the money to just go buying another pair (or something else), so I settled and started to enjoy my er-6i well enough for traveling out and about.
Anyways back on topic I always thought it would be interested to take a pair of stock IEMs with an attached cable and make them detachable. The only ways I've come up with (while doing very little searching on the topic since I've been more concerned with other audio problems on my hands) are as follows. The first is to salvage the 2 pin set up out of a pair of westone's, ultimate ears, or another company's iems with detachable cable already. So if the iems are fried, try opening them up, repair if can, or try to take the 2 pin female mounting plate to put in another pair of iems. The second idea I've had, since often my cable problems come because I'll walk around with one iem in and one not, is to solder a cable in to the iem itself, and terminate it with some male connector that was reasonably small. Do this for both sides. Then at the y split terminate it with two of the opposite female connectors (one for left and one for right) for the males to plug in. So that way if I want both, I can plug both in, or if I just wanted one, I could just plug one in. Then again I'm sure both ideas are fundamentally flawed, though a guy can dream.