Can you give us more info? I don't know what version of Premiere Pro you are using, nor anything about your system setup.
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Able to repro. Bug has been logged.
I'm experiencing this same issue, but in my case, I'm not even attempting to copy clips between project files.
The audio of copies of merged clips won't play in the Source panel. (The audio for each clip should be a single mono track.) Looking at Modify > Audio Channels..., it shows the Source Channel as Channel 1. If I click on that area, I can actually preview the audio using that mini player in the window. So it's in there somewhere. However, there are no other options. Nothing seems to bring it back in the Source viewer.
As the OP described, if I drop the merged clip into a Sequence, I can choose Audio Channels... and switch the Source Channel from None to Left to get the audio to play. Why it's on None, I don't know. And why the options are now Left and Right, I don't know. All I know is that I either have to recreate a day's worth of work breaking down these interviews, or I have to drop all of my clips in Sequences to preview them and somehow work from there.
I'm having this exact same problem. The audio from the original clips play just just. The merged clips do not play in the preview or timeline. I can go into audio channels and hear the audio through the preview however.
The sound was synced in premiere 5.5 and given to me as I project that I need to combine with my master project. Has anybody found a fix?