hi,

I have all the 'alsa' stuff installed;  I tried to replace it with
 OSS4, but the install of liboss4-salsa-asound2 utterly failed.

I see that  liboss4-salsa-asound2 provides libasound2
but it seems that this is not enough.

IMHO the problem is that 'Provides' do not satisfy a versioned 'Depends':
«If a relationship field has a version number attached, only
 real packages will be considered to see whether the relationship is satisfied»
http://www.debian.org/doc/debian-policy/ch-relationships.html

So your 'Provides' is not working as you expect.

You should use a different approach, divert the ALSA library
and substitute with your link; and remove the 'Conflicts' line.