I have my MX300 on PCI Slot 1. This means that it is sharing IRQ with Viper 770 AGP card. I thought I read somewhere that the video card should not be sharing. Should I put the soundcard in another PCI slot?

What motherboard do you have?
As far as PCI sharing, by the way, it refers to the PCI channels that the mobo assigns to each PCI slot (Referred to as A, B, C, and D). Basically, PCI was originally only designed to accomodate four devices, but when mobo makers started adding more slots, they'd just tie in the slots with other resources. On my Abit BP-6, for instance, the PCI-1 and AGP share a host mapping, PCI-2 is isolated, PCI-3 shares with the onboard IDE controller, and PCI-4 and PCI-5 share the same control signal. That means that if I want to use PCI-3, it'll have to work around the onboard IDE controller (Okay since I have it disabled and I'm using a Promise controller there). Also, PCI-4 and PCI-5 will cause problems if I put two cards there that both use the bus-mastering host control signal. Some cards, like the Voodoo2, do not use the host control signal, but instead arbitrate down, so they can be used in tandem with another card in those two slots, but generally speaking, on my motherboard, if I use AGP, I can only use PCI-2, PCI-3, and either PCI-4 or PCI-5.
Sucks, huh?
More information than you wanted, but hey. Some people like to know how things work.

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"And knowing is half the battle!"

I have the Abit BE6 motherboard (not BE6 II). I've got the Viper 770 on AGP, MX300 on PCI 1. Network card on PCI 3 and a 56 modem (which I no longer use) on ISA. MX has been in PCI l for about a year. My Promise 66 controller (which was built on motherboard) is sharing on PCI 3 with the network card. I just read in my motherboard that PCI 2 doesn't share with anyone. It makes sense to move the soundcard to PCI 2 but if I've had problems, I'm not aware of them.