#1, Get a lawyer.
Seriously, if it was a head clog during recording, that broke free and cleaned up later, what you have is ALL you have, period. By playing the tape on a different unit, you pretty much establish that it was a recording problem and not just a playback problem.
One reason I finally got out of the wedding biz was I was not going to commit to adding the safety redundancy recording gear that would allow me to be 100 percent confident in my product, yet still be affordable to my customer base and make a reasonable margin for my lost weekends.
If you want to continue in the biz, backups are no longer a luxury, IMO, but a necessity, for this kind of “act of God” stuff… Hard drives, external audio recorders, etc, these are not to be considered crutches to fall back on or excuses to allow you to do sloppy work, but as safety backstops when something unforseeable or random just happens to an otherwise well-planned and provisioned shoot. The barn only burns down once, you don’t get re-dos in wedding and event videos. The stress of that took quite a toll on me, and after surviving each event without a failure of this kind, I swore it was not worth continuing for the money I was making… Anybody that does this work for long enough is going to have it happen eventually to them, it’s only a question of when and what your backup strategy is to anticipate and handle it. And budget and charge for it commensurately.
This is one of the reasons the better pros cost more: they make the extra investment to protect the quality of the work and thus their business reputation, which lets them. A cut-rate guy trusts to luck and pockets the extra profit margin. 99 times out of 100, he may get away with it. Not saying YOU are cut-rate, mind you, just making an analogy.
Just as a guess, I’d imagine you had something happened to the mic plug or cord, or that a setting switch line line/mic accidentally got changed temporarily during the shoot, or a battery went down. That you have *some* kind of audio, even if low, may offer a glimmer of hope. You can try stacking many synched copies of the track in the NLE timeline and appying moderate normalization to each of them, to kick up the level. You’ll then probably have to apply downward expansion or other noise gating plug-ins to clean it up from there.
Finally, you will probably have to refund the couple in full, and eat the cost of the fixes. It’s the honorable thing to do, and the only thing to do to try and salvage your reputation.