Making headphones mono - FASTER TECH NEWS AND REVIEWS

I recently received an e-mail from a reader, Raul, whose daughter is deaf in one ear. She loves music, but with headphones she'd only get one channel.
While my initial thought was for a fairly simple fix, as I researched to be sure, it turned out to be more complex than I figured.
A short and long answer, to follow.
The short answer is this: a stereo to mono adapter. Specifically, and this is the important part, a stereo male to a mono female. As in, the part that plugs into the iPod is stereo (with two plastic rings, as seen above). These can be found in the form of an adapter plug, or a "Y" cable.
The opposite (mono male to stereo female) is far more common, but for what we're talking about it's not what we're looking for. Also, most "stereo to mono" Y cables merely split the signal, so each mono female end is getting just one channel.
The long answer
What has to happen is the stereo channels have to be mixed down to a single channel, and that channel must be sent to both ears. This way, Raul's daughter, and anyone else wanting both channels in one ear, can get all the music. There are plugs that do this, and cables. RadioShack, Amazon, and Monoprice all have options. Reading through the comments on Amazon, it seems there are people who got this to work exactly as we're discussing.
However, they don't always work like that. There are multiple ways for an adapter like this to work (or not work, depending on how you look at it). The simplest way is to mix the stereo channels down into one channel, so one side of the headphone gets both channels. That's a lot more power getting sent to one earphone, so you'll likely have to turn down the volume. Also, if that's not the ear/side you want/need to use, you're out of luck.
For example, I asked Monoprice about theirs, and this is what Albert Cardenas, product manager for consumer audio, had to say:
"On PID 7160, it does 'combine' the stereo signal into a summed single signal. However, it will only send the summed signal to the left side of the headphones."
Worse, many seem to be poorly built. Also on Amazon, I found myriad complaints on the various plugs from several different companies, ranging from poor build quality to not working as intended.