AFAICT, there are three audio hacks out there:
Hack 1: Use pins 26, 17, and 21 of LM4835 for headphone out.
IIRC, this is a "use the speaker outs".
Hack 2: Use pins 9, 14 (GND), and 13 of LM4835 through a
.33uF capacitor in series with the outputs (to remove
the 2.5V vias) and connect to headphones. These are
the "docking port" pins. People have reported poor
bass response with this hack and recommend larger caps
to make it work better.
Hack 3: A newer hack posted on the BBS. Take the audio signals
straight from the YMF715 and pass them through a TL082
buffer amp. The goal is to get something that'll work
with high-impedance devices (stereos) instead of low-
impedance devices like headphones.
I've got results from two of the hacks and two headphones. I measured
the resistance from GND to one connector of the headphones and used
this as a proxy for impedance. I know that's not quite "right", but
it's probably a decent ballpark :)
Phones 1: 18 ohms. Generic cheapie headphones.
Phones 2: 36-500ish ohms. Has an inline volume control pot.
HACK 3:
-------
Hack 3, Phones 1:
- Sounded like total crap. Extremely high levels of distortion.
Hack 3, Phones 2:
- Sounded like crap. Distortion dropped as impedance increased,
of course, volume dropped too 
Hack 3, Stereo:
- Untested. I just realized all my music comes from my computer :)
I'll have to dig out my stereo to do this test.
Hack 3's audio quality sounded the same whether I powered the TL072
with +5V or +12V. My hunch is that the TL072, not the hack, was to
blame.
I'm guessing the TL072 isn't designed to drive low loads, and that a
beefier op-amp would have had better headphone results. Can anyone
who actually knows what they're doing comment? 
HACK 2:
-------
Hack 2, Phones 1 and 2, .33uF:
- Very poor bass response.
- Bass response degraded (on Phones 2) as I turned up the volume and
decreased the impedance.
Hack 2, Phones 2, 1.0uF, 2.0uF, 4.7uF, 5.7uF:
- Larger caps (small caps in parallel) improve bass response.
- Higher volume (lower impedance) is possible without degraded bass
response as the filter cap gets bigger.
- With sufficiently large caps, lower volume (high impedance) can
result in slightly muffled sound.
- Under Windoze, you can compensate for some of this by clicking on
the speaker icon -> Properties -> Advanced, and playing with the
bass and treble settings.
With both the Bass/Treble settings near their maximum levels, my
untrained ear got decent (IMHO) performance across a wide range of
volume levels with a 4.7uF cap. This gave me reasonable sound with
the 18-ohm "phones 1" provided I turned down the master volume by
about 25%, and good sound on the "quieter" (36-500ish ohm) headphone
set. I believe the sweet spot for "kinda loud but not deafening but
you don't have much bass fade-out or tunnel-sound" was around 350 ohms.
I'll attempt to duplicate these tests with a stereo when I get the
thing unpacked, and/or try it with a few friends' boom-boxes.
One interesting thing - assuming you're not gonna run both a stereo
*and* headphones at the same time, maybe you can use Hack #3 to give
you a buffered line-out into your stereo and Hack #2 for a headphone
jack. They're sufficiently independent that you should be able to
build both into the IO.
Any suggestions where to go from here? Or other folks' results?
( *** As for this stuff -- don't take my comments on Hack #3 - the op-amp -- as criticism. I know Hack #3 isn't really designed for headphones... but those 10K resistor ideas intrigue me... I still have more testing to do but figured I'd share what I know now...
