I've been doing this for many years, bringing the outside sounds
inside for my inside only cats. The easiest way is to use a
baby/infant room monitor. You put the microphone part outside a
window or door where it won't get wet, and put the receiver part
inside your room/house. If you know an electronics techie friend, he
(or she) can easily connect the receiver to a powered computer
speaker. That makes a much better sound.

My new male cat, kitty has been here about one year. At the moment he
is lying less than one foot from the speaker with his ear pointing at
it. I think he likes it. They all do. Sometimes it takes some
perception, other times it is easy to tell. Keeping the volume very
low at first is a good idea.

Don't expect inexpensive room monitors to transmit more than 20 or 30
feet. My cheap Fisher-Price room monitor is clear at short range, and
sounds great connected to a computer speaker.

Have fun.