In medium-large trials - >1000 frames - at some point, the sound and music selection inputs in any frame editor row start failing, particularly after a large part of that trial has been scrolled through and browser performance starts taking a hit. What happens is that the select's options simply won't appear when it is clicked - the container that select was in simply ends up blank, and no options can be selected. Clicking on another part of the page brings back the select itself, but it often takes many attempts to actually make the options pop up, which ends up being a real pain the bigger the trial gets.
This happens to me in the editor for every trial eventually, though I can definitely confirm that it pops up in this specific one. There don't seem to be any steps to reliably reproduce the problem, since it tends to pop up randomly - and the amount of attempts it takes until the options finally do pop up varies as well. It's confirmed to occur in Google Chrome in version 53, on a machine running Windows 10 Pro, with all relevant plugins up-to-date.
Here's a video of the bug in action.