Ten Thousand Dream Interpretation

by Gustavus Hindman Miller
To dream of being dumb, indicates your inability to persuade others into your mode of thinking, and using them for your profit by your glibness of tongue.

To the dumb, it denotes false friends.

The Complete Guide to Interpreting Your Dreams

by Stearn Robinson - Tom Corbett
Whether it was yourself or others who were unable to speak, a dream of dumbness is a warning to keep your business to yourself and avoid any speculative ventures for the time being.

New American Dream Dictionary

by Joan Seaman - Tom Philbin
1. A need to keep affairs private, keep opinions to oneself.

2. Difficulty with emotional expression.

3. Possible arguments in the making—possibly domestic, sometimes social.