ya know just to throw in my $.02, The sound blaster live series of cards do not handle increased PCI buses due to overclocking the PCI bus, this is because the chips cannot handle running at the speed the pci bus is transmitting at, and therefore overheats the card causing it to fail (in mild situations).
Increasing the PCI bus of your cards does, in fact, increase the performance of your cards, if there is a bottleneck at the PCI bus, the Voodoo5 5500 AGP sees SIGNIFICANT increases in performance when the AGP bus exceeds 66mhz (83mhz stable) because the dual chip architecture actually has to wait for the agp bus to transmit the data to other devices within the computer (cpu, memory, etc.) and i have found this to be true through extensive testing with 5 3d games and 3dmark2001.
I can send more info if you need it.
I feel that soundcards dont see a performance increase because they dont have a bottleneck at the pci bus, your computer never has to wait to load a sound stream because your soundcard is too slow, but if you have a high end card like an Audigy or something, you may be able to measure a performance increase with large amounts of data like 96kbps 48000hz 64bit streams of sound and such with effects, or playing multiple streams at the same time. Im outta my league when it comes to soundcards, but i have done extensive experimenting with fsb overclocking.
If there is some kind of compression going on with modems, it is VERY possible that performance could be increased by increasing the PCI bus.