Just thought I'd post this as I've not found any similar information by Googling "Groups". (Lots of "it's not ethical to meddle" stuff, of course...)
I've had an NHS behind-the-ear hearing aid for the last few years. It's an analog type (using a 13ZA cell with an on-T-off switch and rotary gain control marked from 1 to 4) and this morning it was dead, despite it having had a new cell fitted a few days ago. It was still dead after I tried a new cell in it, so do I take it in to my local hospital today for repair and be without it over the holiday weekend, or do I see if there's anything obviously wrong with it? Yesterday I had noticed that it wasn't as good as it used to be, or was that my hearing continuing to get worse...
The tube to the earpiece was clear, but when I examined the microphone area through a magnifier I was horrified at all the crud which had accumulated, and after removing a fair amount mechanically, I finished off blasting the hole with IPA from an aerosol using the fine tube applicator. The visual difference was impressive, but the hearing aid was still dead. So I dried the HA on top of my hot water tank for 15 minutes, and it's now as good as new. I can even turn the gain control down a notch. My guess is that the crud had built up and stopped the microphone diaphragm from vibrating.
I've no experience with any other HA type, and I'm not taking any responsibility if your ruin yours; but in my case I've saved two hospital car park fees, some petrol and I've got my hearing back 3-4 days earlier than otherwise.