I'm not sure. I have both PJ64 and 1964 in the same directory and so any files required to work with PJ64 were already there I think. Also, I am using the special audio fix version Azimer released to work with his last audio plugin. Dunno if that makes a difference. Try it. Just extract to your PJ64 directory and move the plugin to your plugin folder. I believe in order to Rice's plugin you have to msvcp71.dll and msvcr71.dll in your PJ64 directory as well, but I'm guessing you already do since you said it works, just not well (I think it was you?).
[Update]: I just tried PJ64 1.5 and yes it does make a difference. The symptoms I was getting with 1964 are also in PJ64 1.5. The difference my be that the audiofix version is based on the 1.4 source. Hmm.