I need an external audio switching box with a simple ASCII control protocol
(Serial port / RS232) that will allow me to switch (under computer
control) multiple analog audio feeds. The box should be able to accept
at least four (4) audio inputs and switch to one outgoing feed,
suitable for routing into a PC's sound-card line-in jack. A nice to
have extra feature would be the ability to route any two of the feeds
into the left and right audio channels for easy A/B comparison. Does
such a serial-port controlled box exist?

Hi

I'm assuming from the A/B comparison question you mean 4 stereo inputs.
There are some. This kind of switch with RS-232 control is much more
common these days in integrated AV/switches. However for just audio
switches here are a couple:
http://www.extron.com/product/product.asp?id=mls100a&subtype=14&view=over#tabs
http://www.tieline.com/Tieline_04/Prosolutions/AS400.html
I'm sure there are others around. The type of A/B comparison you want
is unlikely to be found.
The inclusion of RS-232 control tends to mean these are products aimed
at the pro market and they are priced accordingly. If you do not
require RS-232 control specifically, but more the ability to control
from your computer, for the same kind of money you might be far better
off opting for a multi-channel sound card which will do everyhing you
want and far more to boot, including the ability to set up the kind of
A/B comparison you want.

Clarification on A/B comparison. I'm referring to 2 mono feeds (HF
radios) where the left channel would be radio "A" and the right
channel radio "B". The purpose of this audio comparison is "diversity
reception" where radio "A" is on one antenna system - say a dipole -
and radio "B" might be on a beam antenna. In HF (0-30 MHz) radio two
antennas can produce radically different results, especially for DX or
long distance reception.

Since this application is for remote control over the Internet, serial
port control is essential. A general purpose server program to control
serial devices has already been programmed in Python by me.

Since the app is remote audio control pre se, there is no need fpr a
full-blown KVM switch.

I will certainly look at the web sites you mentioned. Thanks.

Almost definatively it does exist. Search for "audio switch equipment".

Holy Cow!   I'm looking for almost exactly the same thing, and have
also been having a hell of a time finding one (well, one that isn't
price professionally, i have about a fifty dollar limit).   But
ah...goodluck!