To answer my own question: so far as I can tell, you can do this using wired headphones, but you can't do it with made-for-iphone hearing aids. :-(
Here's the approach I tried:
- Get hold of a USB audio device that has sufficiently low power requirements that it can be powered by the iPhone's Lightning port. I chose a Behringer UCA202, which has stereo RCA analogue inputs (and can also act as a DAC, although that needn't concern us here).
- Get a "Lightning to USB Camera Adapter" cable.
- Download some sort of sound recording app for the iPhone, for example Voice Record Pro.
Now, let's say I disable hearing-aid streaming, connect the Behringer to the Lightning port via the Lightning to USB Camera Adapter cable, run the recording app, hit REC, then go into the sound-level check (which allows you to input sound and monitor it without actually creating a recording); then I connect a pair of wired headphones into the iPhone's headphone socket. Then (with a little juggling of settings in the app) it *is* possible to monitor the incoming sound over the headphones: you've got stereo sound going into the Behringer, the phone pulls it through the Lightning port and gives it to the app, the app sends the stereo signal out over the headphones, and it's all working.
So, problem solved, right?
Wrong. 😠
If you now activate hearing-aid streaming, for some insane reason, this changes the way that the iPhone receives sound from the Behringer device! What you now see in the recording app is no longer the result of capturing both of the incoming stereo channels; instead, it captures just one of the two channels, and feeds that to both ears! So, although it is still pulling sound in through the Lightning port, and is sending it to the hearing aids, you can only pull in one of the two stereo channels. Switch off hearing-aid streaming, restart everything, start recording, and plug in wired headphones, and it's capturing in stereo again. 😕
I tried this with more than one recording app, and they all did the same thing, so clearly it's the phone itself that's going haywire, not the app.
Notes:
- Use of a "Lightning to USB Camera Adapter 3" cable allows the iPhone to be charged while the device is connected to the Lightning port, and may possibly also allow the use of devices that require more power than the Lightning port can provide. Device power requirements can also be dealt with by using a mains-powered USB hub; but the phone can't charge from that while something is connected to the Lightning port.
- There are devices designed to connect directly to the Lightning port, for example the Line 6 Sound Port; but I haven't experimented with them (and the FAQ for that device specifically says you can't monitor sound from it by Bluetooth because of latency issues).
- If for some reason you do want to capture only mono sound, there is an easier way to do this: you can get a cable with built-in attenuation that connects to the iPhone headphone jack (and mimics an external microphone).
- I haven't tried this, but if you want to try a stereo sound-capture device that has a digital (rather than analogue) input, the Hifime UX1 might well work.
There is one rather clunky work-around....
There's an app called RemoteSound (full version is 79p, trial version is free). What this does is allow an iPhone to act as a wireless sound-card for a Windows PC. You download a separate Windows application and run it on the PC, then launch the app on the phone, and the two connect to one another over the Wi-Fi. (The PC needs to be connected to the same local network as the phone). Any sound sent to the PC's default sound driver is also sent over the network to the phone, and you hear it played there.
This *does* work in stereo with hearing-aid-streaming enabled; the sound quality is comparable to what you get streaming directly from the phone; and, while there is some latency, it's not as bad as you might expect. However, this only works well if there isn't too much other traffic on the network; if there is, the sound can break up. (There's an option to compress and/or buffer the sound, which makes it less prone to break-up, but also increases latency).
In theory you could feed sound from the TV into the PC, and from there send it to the phone via RemoteSound; but that isn't exactly portable!
A final possibility for some users might be the Tunity app: you point your phone's camera at a TV screen, and it identifies the channel you're watching, and then streams the TV sound live over the Net. But this only works for some channels, and only in certain countries (and I think the UK isn't one of them).
