I like sound effects for a layout...
in moderation. Sound equipped locomotives are fun for about 10 minutes and then start to get on my nerves. Most people have them turned up too loud anyway. Other sound effects like car horns, crossing gates, animal noise, etc. can also be enjoyable in the right setting but are easily overdone. I think they should be just barely heard, not in our face.
On my old HO layout, I had purchased the MRC City and Country Sound Station and the MRC Sound Station 312. Both are now about 20 years old, and both are simple sound systems with a handheld controller that features about 18 buttons including a couple of on/off slide switches, a power switch, and a volume knob. They are hardwired into a pair of speakers, and my version of the Sound Station 312 has the power supply also hardwired in (the City and Country one has the power cord on a plug that disconnects from the back of the speaker).
They are pretty basic but I enjoy them. I actually own three of each, with the first being used on my N scale layout and the second used on my HO layout. The third pair are new in box from my local train store that I bought during a going out of business sale for $5 each. Not bad.
For my O scale layout I wanted an easy way to mount the speakers underneath. They have no mounting holes or brackets and are designed to just sit on something. So, I built some L-frames out of scrap lumber. Then, the speakers were cinch-tied in place to the wood. A pair of metal L-brackets from my stash were attached to each wooden frame and then they were screwed to the underside of the layout's plywood.
Since the speakers are physically hard-wired together, I had to move it all as a unit and that made installation a bit awkward. But it worked. I could always cut the wires and then splice in longer ones in the future, but it isn't a problem now.
I mounted the Sound Station 312 under my left section of benchwork because it features railroad and automobile sounds. That is where my road crossing with flashers will go so it seemed to make sense. The City and Country system with animal sounds is being reserved for another portion of the layout that will feature a farm.
The sound isn't as crisp as modern sound systems and the sounds are just generic noises but that's okay. It makes Harrison and me happy, which is all that really matters.