I read the 2006 Modern Library edition. It was translated by Natasha S. Russell and in her introduction she says the following:
Of particular interest to language fiends, and to this translator, is Zamyatin’s relationship to the sounds of words. He told the artist Yuri Annenkov of the qualities he ascribes to certain sounds and letters. L is pale, cold, light blue, liquid, light. R is loud, bright, red, hot, fast. N is tender, snow, sky, night. D or T is stifling, grave, foggy, obscuring, stagnant. M is kind, soft, motherly, sea-like. A is wide, distant, ocean, misty mirage, breadth of scope. O is high, deep, sea-like, bosom. I is close, low, pressing.
The information quoted by Hamlet53 from Wikipedia appears to be sourced from this same introduction.