The question is why do a lot of the modern CDs sound so bad from a sound
quality side, a lot of the CD's have PC CD-ROM sections is this causing
the quality issues or is it just sloppy mastering?

It may not be either of these things.

With so many low budget project studios being used these days,
with poor monitoring, it may be that the CD is no worse than
the original:-)


As we have discussed before on this group, the role of
the mastering engineer, be it disc, CD, or whatever,
seems to have changed.
Previously, the objective was to make the finished product sound
as close as possible to the original allowing for the medium in
which he was working. The old saying "It takes considerable
skill to make it sound the same, but any fool can make it sound
different" is as true now at it was then.

The mastering engineer was left to his work, and responsible
only to the producer, in co-ordination with the recording engineer.

But, unfortunately these days, the master tape is so often regarded
as some kind of a "half-way" stage, instead of a finished product to
be cloned as accurately as possible.

I have often been disappointed with test CD's of projects
on which I have worked as an engineer, and always try to attend
the mastering session.

Several times a year I am ask to attend mastering sessions for recordings
with which I have not been involved, as a kind of referee:-((

Usually, each and every member of the band wants to be there too, and
at this late stage they start to discuss/request things that should have
been discussed/requested at the mix, or even during the
recording process.

The answer to your question may well be:
"Too many cooks........"

Iain