Assuming 16-bit, 44.1 KHz, stereo audio, that's just 176,400 bytes/second of decompressed audio data on the bus, well below what ISA can handle. With a 430FX chipset, I'm guessing you're using a PCI graphics card and the on-board IDE controller, which is also PCI, so you shouldn't have much contention for the ISA bus.
Instead, I wonder if this has something to do with DirectSound acceleration or sound mixing. The driver that ships with the OS for the ES1869F might not support DirectSound. DirectSound is supposed to have lower CPU utilization than WaveOut. See what WinAmp is using on these cards.
Also, I don't think the ES1869F supports hardware mixing of multiple PCM audio streams, while the AudioPCI does. Software mixing is taxing on slower CPUs.