Well, actually, I think if it's stuttering, it's probably due to too much CPU usage rather than the sound driver's inability to interleave sounds.

Take a look at this old post. This kernel patch may not have been included in Hijack, but if you patched it in, I think you'd have better results. PCMplay would just have to open the audio device with O_SYNC.

Oh, and to see how much memory flite is using, run the following from a shell prompt when flite is running:

ps -eorss,pcpu,pid,args | fgrep flite

The first column will be the RSS, or resident size, in kilobytes.