If this is a phase problem on a mono channel where sound from two mics (or sound reflected from the desk back to the mic) mix and cancel out certain frequencies, you can try some eq to help the mix. The frequency range between 250 and 800Hz is where mudiness often occurs. Start around 300Hz and add or subtract acrossm the mudiness range and see what happens.
If the mics are on separate channels, try what Drew suggested or try reversing the phase on one channel and see if that affects the mix.
In the future, get a good set of phones and listen carefully on the set for phase cancellation. You can rarely get rid of it completely once it’s recorded.
John
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