I can't get to the audio recording from here but from your description, it sounds like the timing could possibly be slow.
Reving it up utilizes the vacuum advance to bring the timing up some.
Do you have a timing light? If not, you could do as I've done for years and slightly loosen the distributor where you can turn it by hand ever so slightly while it's running to advance the timing a little, Rev it up till you hit the sweet spot then lock her down again.
Jerry W.
