how to analyze audio output

[baudline]

Someone asked "How do I get baudline to analyze the audio output going
to my computer speakers?" This was such a great question that I added
it to the FAQ http://baudline.com/faq.html#loopback Here is the
answer:

What you need is a loopback. There are several different ways to
accomplish this:

1) Use a loopback Y-cable to connect your audio card's output to its
input. This analog solution will also be measuring your audio card's
SNR and other distortions. Some devices such as the Edirol UA-25 let
you do this loopback digitally with the push of a button.

2) Go into baudline's Input Devices window and select the channel
called "volume". Selecting this channel instructs the /dev/mixer of
the audio card to do an internal loopback. Unfortunately this option
has the same analog issues mentioned above and not all audio cards
have the "volume" mixer channel.

3) Use JACK and route the audio output source to baudline's input
sink.

4) Use SoundFlower on Mac OS X.

5) If you are using baudline's Tone Generator then there is a digital
tone loopback button in the Input Devices window. This option is great
for DUT measurements such as performing dual-channel transforms like
the impulse response and transfer function.

Can you think of any other ways of doing a loopback on FreeBSD, Linux,
Mac OS X, or Solaris?