More than 100 audio files sounds like a lot for the PCIbex Farm’s capacity. Your experiment will have to send one request per audio file to the servers, and since the PCIbex Farm servers are doing poorly these days, it’s not much of a surprise that preloading is slow. Some of your requests probably take time to be accepted, some of them maybe even never succeed.
If you can do it, I highly recommend that you package your audio files into a single zip file that you upload to your servers/s3 bucket. The procedure is described on this page: https://www.pcibex.net/wiki/zipped-resources/
Another thing to be aware of is that, at the moment, Safari doubles up on the many-request issue, it just doesn’t like handling many requests at once.