Nostalgic things
There is also a personal retro computing nostalgic factor that may feel kind of weird for you, but still. Back in the 1980s electronics and computer related stuff in USSR mostly were both rarity and luxury. People weren't yet much familiar to home computers, electronic games, video recorders, TVs with remote, FM radios, and such, it all was a technological novelty coming directly from foreign movies and sci-fi books. In the very late 80s these things started to get imported and appear on the black market under insane prices, and with the fall of USSR all kinds of imported and local built electronics flooded the country - you probably have heard of Spectrum clones and Dendy, as few examples of this.
For us kids of the time all kinds of previously unseen technology were extremely fascinating. Casio watches with melodies, Chinese sound effect keychains, Game&Watch clones, Brick Game (Tetris electronic game) - everything caught our curiosity and attention. We loved to explore its capabilities, disassemble it to see what is inside, and so on. AONs became part of this too. Besides the ever-fascinating clever use of the 7-segment LED display that used to produce cryptic text messages (Ln Hold), one thing we appreciated a lot was the sound. Right from the excerpt from the Mikhail Glinka's 'Glory' that played during power up sequence of the Rus firmware (originally a choir piece that sings 'Glory to Russia', akin to the firmware name that is short for Russia), to a few dozens of built-in ring tones and alarm melodies, which were versions of classical and popular music. We listened and loved to recognize the famous songs in this basic square tone computer renditions. As you may guess, with Z80 and 8253 on board it was all monophonic 1-bit square wave music, albeit nicely programmed. One missing historic piece that alongside the Speccy and NES music got many of us here interested in computer music in general.
A bit later the Rus firmware was upgraded with speech synthesis - male then female voice that could say the incoming call phone number, or current time, and some other things, constructed of pre-recorded words, like old talking clocks did. It also could be used to know who was calling when you're not at home, you could call to your own number and use tone dial phone (or portable DTMF beeper device even!) to enter password and hear the speech synthesizer talking you through the call list. It was very impressive at every level, and voice quality was pretty nice for the time. Think of how voice clips were impressive in early computer games.
Close friend of mine at the time (mid 1990s) was involved into production of AONs, his father ran that private business, so he helped with soldering and assembling a ton of them. Thanks to this I learned a lot about the internals, and was also fixing and upgrading AONs for my friends time to time. So it provided me with some early digital electronics skills.
I first got the idea to use the hardware to produce some sounds back in 2014, when I was thinking of creating a simple 1-bit DIY synth and was considering to use Z80. I thought I still owned a pair of old AONs that used to belong to my childhood friends in 90s, and did some research, planning, and programming of basic emulator to make further development more comfortable, with plans to create a simple 1-bit programmable drum machine out of it at first, and something more interesing eventually. Unfortunately, later it turned out both of these somehow went to trash during major cleanup at house at some point in 2013, and I havent't found another one. Then other stuff got priority, so I had to postpone the idea. Until now!
You can listen to some of the monophonic music here: https://www.youtube.com/watch?v=A0IKdIRfoFw. This is Rus-25 on the 80C31-based board, but the same music was in the Z80 version as well (Rus firmware went to 23rd revision there, then moved to the new HW).
You can hear the speech synthesiser in this boring and shaky video, as well as see some of the interface. Again some of 80C31-based versions. https://www.youtube.com/watch?v=SOHX96BbhCk