Interests
In our digital era, the sound reproduction is nearly ideal : 16 bits at 44.1 kHz is enough for all domestic uses (my 2 cts).
Professionnal needs are different because of all the processes of mixing, EQ, etc,… that degrade the overall quality : recording and processing may benefit from being done at a higher bitdepth (ie 24 bits).
So if the electronic path from recording to listening is of high enough quality, what are the fields were improvement is still to be expected ?
– spatial reproduction is a very interesting theme from recording to reproduction. Multi-channel audio, WFS, binaural and other stereo techniques such as stereo dipole, are important fields of research.
– soundfield in a room : walls have a strong influence on sound quality in domestic conditions. This depends on the loudspeaker polar radiation. Some speakers are near from “perfection” in direct soundfield but as they don’t radiate uniformly to wall directions, the result at the listener’s ears is difficult to assess. Digital room correction and acoustic treatments can improve reproduction but are difficult to setup.
– headphone equalisation : the last step to perfect reproduction ? but you also need a perfect recording adapted to binaural listening (and no head movements !)
An some brutal words for audiophiles who may arrive here by pure hazard,
I do believe that :
– the main reason for bad quality audio is that most CD’s are badly mastered, over-compressed and too near full-scale.
– all well-designed amplifiers sound the same,
– all well-designed CD/DVD players sound the same,
– cables have no influence on sound quality in normal conditions and standard length, apart too thin speaker cables
– amps with valves are old-fashioned and generally bring only some harmonic distortion, noise and maybe frequency response limitations,
– good CD’s sound much better than LP’s,
– there is no special need to go higher than 48KHz/24bits in recording/production and 16 or 20bits in reproduction
– the quality of your audio system itself is 99% related to your speakers and room
– few speakers are really well designed
– speakers/room equalisation may be difficult but rewarding
Now back to some interesting topics :
Interesting sites about audio
Siegfried Linkwitz, various themes but mainly about loudspeakers used as dipoles
David Griesinger, another must
Elliot Sound, lots of interesting topics
Kreslowsky’s Music and design
Michael Gerzon fruitfull archives about different audio topics, allways refreshing and worthy. The hosting website has also very very interesting topics.
The AudioCritic, the (only ?) serious webzine about audio
Le site de Laurent G. : acoustique de salles (en francais)
Room correction
Digital room correction and convolvers : a nice list seen at Aurora’s page
Electrostatic loudspeakers
I have great respect for the work of Peter Walker. Even 50 years ago, he was ahead of most of nowadays audio designers.
Coaxial speakers
A way to go for dynamic speakers ? ie Seas loudspeakers
Speakers with electronic feedback
Spatial audio
Binaural
Loudspeakers directivity
Ambisonic
HRTF
Dummy head recording
Recording and microphones
Cables for audio … no, it’s a joke 
Measurements
ABX and DBT
ABX, is the base of DBT double blind testing, the mother of true and fair testing in many fields (from medicines to audio)
PCABX, software and samples to try for yourself
ABCHR is a way to compare and rate quality
Plugins creation
Plugins with Synthedit or Synthmaker
Headphones
Crossfeed
Diffuse field EQ
Rockbox
Rockbox forum
Audio forums on internet
most forums are full of stupidities but hydrogenaudio forum sometimes has interesting topics
Also have a look at diyAudio
Recommended literature
first is AES : preprints, journal and proceedings
John Borwick, Loudspeaker and headphone handbook
this is really a must have, the chapters on electrostatic speakers and the one on heaphones have, I think, no equivalent in other books
Francis Rumsey, Spatial audio