Hey- it's not illegal everywhere! Here in NZ & Australia we have 2 free UHF CB channels (~470 MHz) reserved for telemetry/telecommand- although of just a few seconds per hour. This is a source of some local griping, especially since it's considered the "per hour" was a type & should have been "per minute"!
IF it's locally legal, all manner of info can be sent, with even the likes of SSTV (Slow Scan TV), Hellschreiber etc a doodle. Decoding can be via PC sound card software. Radio hams have been especially active in recent decades with new data modes. If you're really keen about these things then consider getting your ham licence too! The traditional Morse requirement has now been largely scrapped, & anyone with an electrotechnical mind would find the licence test straight forward.
For simple audio data use even weak transmissions at the free low end of the 88-108 FM band may be handy. However 433 MHz ISM is certainly THE slot to initially consider, & ranges using simple Yagi antenna can be 100s of metres. Superior 433 MHz gear ( HopeRf, Yishi etc) will go for miles line of sight. This band is often very noisy in urban areas due to numerous other users - hams, wireless doorbells, weather stations, energy meters, security, toys, garage door openers etc.
Way down at 27 MHz MHz & the like -the CB spectrum slot once VERY ugly but now almost deserted- all manner of weak free transmissions are tolerated globally. Kids "~US$5 a pair" 27.145 MHz walkie talkies & cheap R/C models in fact may suit parts hacking for more decent tone/data & control use.
Stan.