Quality is varying, of course, but some are surprising me at their age. This one I picked just because it is the closest I got to 100 years (Victor Concert Orchestra playing Dvořák in December 1912): http://www.loc.gov/jukebox/recordings/detail/id/2988 (http://www.loc.gov/jukebox/recordings/detail/id/2988)
This one sounds more like expected. Metropolitan playing Waldteufel, 1902: http://www.loc.gov/jukebox/recordings/detail/id/5388 (http://www.loc.gov/jukebox/recordings/detail/id/5388)
Vocals, that is always a test. Janpolski sings Grieg in 1910, accompanied by orchestra, hiss and VU meters in the red – still not hopeless! http://www.loc.gov/jukebox/recordings/detail/id/2045 (http://www.loc.gov/jukebox/recordings/detail/id/2045)
So glad the loc has been doing this. From what i've read they are taking ancient acetates and using some kind of transfer to paper to preserve these ancient recordings. Amazing. Dunno if this applies but oddly enough the loc has classified Public Enemy's Fear of a Black Planet as an 'important historical recording' http://www.loc.gov/rr/record/nrpb/registry/nrpb-2004reg.html (http://www.loc.gov/rr/record/nrpb/registry/nrpb-2004reg.html)