Multitasking is all about running several processes at the same time. The N900, via the Linux kernel, does this "beautifully".

But, this doesn't mean that there are unlimited resources for the running processes. And since it's very easy to hear when music stutters, this becomes very obvious when playing music and doing something else at the same time.

It's not as easy to spot that the scrolling done in the browser "stutters" too while the phone is busy with many tasks.

So, this sounds like you need to adjust your expectations a bit. Either you make more resources available (overclocking is one way) or you adjust to how you can use the resources that are left while multitasking.

Being a bit more technical, sound is troublesome on the N900 because of, in my opinion, two things: using PulseAudio and not using the proper scheduling algorithm in the kernel. PulseAudio uses lots of time in the CPU to do all the things it can do (which brings some nice features) and this punishes all other processes.