I realize I'm late to the game here, but I came across this forum and I think I can help you in a way.
I own TinyTx Inc. (tinytxs.com) We’re a Canadian company which strictly manufactures UHF acoustic surveillance devices, the only manufacturers in Canada. Our surveillance transmitters are used by both the private and public sector, including wildlife researchers and ecologists as of late to achieve the same result, 24/7 acoustic surveillance.
Our devices are very small and lightweight and transmit continuous ambient audio upwards of 700-1000 metres for 22 hours (longer times can be achieved with different power options) and the antenna is only 6 inches in length.
We never intended to find a market in wildlife research, we mostly deal with law enforcement, but it turns out that constant audio surveillance of animals of interest plus the ability to use the device for telemetry tracking (since it’s always transmitting audio) has been something quite useful for our clients.
The V1TX would serve your purposes well, it’s our most popular UHF audio surveillance transmitter, it’s priced at $129 but I can give forum members a heavily discounted price if anyone is interested.
Anyway, this forum post interested me and I commend you all for your hard work and research, I just thought I’d extend our service in case you’re still looking for solutions! Feel free to email me directly [email protected]