Honestly if you're interested in this stuff I'd encourage you to get a radio license. You'd probably be interested in repeaters, satellites, and HF, but you might also be really interested in stuff like APRS (which is sort of a giant radio mesh net). I don't know what country you're in, but usually licenses are really cheap and just require you to pass a relatively easy (with some studying) multiple choice exam. The hardest part is usually just figuring out where you can take a test (at least in the USA tests are run by volunteers, so you have to do a little bit of searching to see when they do them in your area).

Did you find Meshenger yet? It's not exactly what you're after as it still relies on a local wifi network for comms, but it's one of the closer apps I've found to what you describe.

That said, what's the app you found that lets you transmit asynchronous voice messages? Sounds kinda cool!