The problem is with your Excel file. There seem to be extra entries that are not actually visible to PsychoPy, but are being read as empty trials by the JS library. To demonstrate, change your practiceLoop to sequential, and you will have 4 practice trials, followed by the beeps. The beeps occur when there are no wav file enties to play, so it falls back on the default (at least, I think that is happening). To fix, copy only the wav file cells in the conditions sheet into a new sheet, being sure not to copy any empty cells and change the conditions file in your loop to point at this new file. Test, then, you can switch back to random trials and you should have random order of your sounds.
@jon, this is the same bug as Online experiment trials loop nested in the block loop not ended correctly from my email on Monday.