The first one isn't so simple since any code loaded into the browser can access the audio capabilities of the system. It is possible to isolate running instances of plugins, but don't expect anything like Windows' per-application audio
settings. Maybe make a proposal to the maintainers of the NS plugin API?
The second one seems doable, but might fall victim to the same problem that Excel has when you tell it to work with "web data": the structure of a page is volatile. That's why RSS is so popular for syndication.
If you put a media player (flash, SL whatever) in a web slice, will it keep playing when not displayed? That's what I was thinking about for idea #1. Of course this would require support from the web site to implement. And so would web slices. I think
whatever Apple call their web slice-like feature in Safari works off of arbitrary divs of web pages though.