when u open the camera slider there is a preemption u want to take a picture or record a video. hence the sound coming from the music player is stopped. This is a design and not a flaw. It has nothing to do with multi tasking which means the ability to run more than one program at the same time. multitasking does not mean every program would run in their optimal level with all functions working because this could lead to chaos . (e.g when you are playing music and a call comes in, the music would be pulsed till your call is ended. so even though your music player is still running doing an active call, some of it functions are suspended because the call is given priority. if this were not the case the N900 would be unusable. If you do not like the fact that the N900 gives priority to the camera over the music player you can always file a bug report or an enhancement request and make your suggestion known.

This is actually a limitation imposed by the underline technology behind application install that the N900 uses. It is based on apt-get which is a frontend to dpkg the GNU Debain/ Linux package manager. Once an installation process is on. dpkg puts a lock on where package cache directory. what this means is that no other package management would be allowed until the installation is complete. one way round this is to have a pakcage manager which support batch installation of programs just like synaptic on debian there is a bug filed on this which doesn't seem to be getting much attention. you might want to help make the devs know people want that feature by voting for it