Unable to log in to podcast's site manager

There's no way for an outsider to know what Apple ID a podcast was submitted under. I don't know what the site manager does - you don't need the ID to update the feed, just access to the feed is sufficient.

If you contact Support they may be able to help, though it's possible that for security reasons they won't tell you the ID. You could alternatively try going to http://appleid.apple.com and clicking 'Find your Apple ID' and see whether that process throws up anything you can try. You will be asked to provide identification, probably the credit card associated with the accont.

I had a further look into the 'Site Manager' - from the rather vague information I could find it looks as if it's only to do with iTunes U, not with ordinary podcasts.

The Store page for your podcast is using the feed at http://feeds.feedburner.com/popuparchive - this appears identical to the Soundcloud feed you link to and is presumably drawn from that; so if you want to add episodes you simply add them to the Soundcloud feed: the last episode you added was on September 15th and this appears in the Store.

What were you trying to do with the Site Manager? if you just want to continue to update your podcasts you proceed as I indicated above.

Do let us know how you get on and, if you get in, what you find there. it's all useful information. I did try logging in myself and though it showed the correct Apple ID it wouldn't accept the perfectly correct password.