Why does my mix sound muddy?

Where mud lives

The low-mids, roughly 200 to 500 Hz, hold the warmth of most instruments. One track with plenty there sounds full. Six tracks with plenty there sound like a blanket over the speakers. Below about 100 Hz the problem has a different name, boom or rumble, and a different cause: low-frequency content on tracks that do not need any.

Find the pair, not the range

Mud is a masking problem between specific tracks. A broad cut on the master removes warmth from everything, including the tracks that were fine. Solo is the wrong tool for finding it, because every track sounds good alone. Listen to the full mix, mute tracks one at a time, and note which mute clears the low-mids. That track and the one it was covering are the pair to treat.

Fixes, in order

1. High-pass what has no low end to give. Vocals, guitars, hi-hats and most synths can lose everything below 80 to 120 Hz without sounding different in the mix.
2. Cut the masker, not the masked. On the less important track of the pair, a broad cut of 2 to 4 dB between 250 and 400 Hz (a Q near 1).
3. Check the reverbs. A reverb return adds low-mid energy from every track sent to it. High-pass the return at 200 to 300 Hz.
4. Look at the arrangement. Two parts playing the same chord in the same octave cannot be separated by EQ. Move one up an octave or thin its voicing.

Judge every change with the whole mix playing.

In Zammoth

The engineer measures the mix in eight bands, reads the low-mid build-up as a mud figure, and measures masking between pairs of tracks band by band, including whether panning already separates them. So the answer names the tracks:

Each EQ band has a range of ±12 dB, and its high-pass filter has slopes from 6 to 96 dB per octave. See the EQ reference and what the engineer can measure.