For years, I have had a microphone outside to bring the outside sounds
inside. At first, it was for security purposes. I noticed my indoor-only
cat liked it. Over the years, the sound quality has improved thru re-making
the device. Also, thru many years of careful observation combined with a
little common sense, I have come to realize my cats (now two) very much
enjoy being able to hear what is going on outside. Going by appearances,
you might not be able to tell. For example, they do not run to the window.
A window is good too IMO but I know they like kitty radio maybe more than
sitting in a window. Leaving a human radio probably is better than nothing,
but I think they do not get nearly as much out of that.

I suppose the concept is simple enough. There are a jillion ways to
impliment the thing. If you have space outside, perhaps the easiest
implimentation is to use a modern infant room monitor. Some do not sound
very good. Mine is hooked up to a powered computer speaker. But if the
infant room monitor has a headphone output, that probably can go directly
to the line input of a computer sound card (1/8" plugs on both ends of the
wire).

So, when you are very concerned about your lonely cat while you are away at
work/wherever, please consider that alternative. Cats love to use their
ears, and there are many interesting sounds in an average neighborhood for
a cat to feast its ears on. If you do so, you might want to introduce it
slowly, very low volume at first.

Good luck.