Moon altered sound system (4)

I believe the answer is:

mono

I'm a little stuck... Click here to teach me more about this clue!

'sound system' is the definition.
'mono' can be an answer for 'system' (thesaurus). I'm not certain of the 'sound' bit.

'moon altered' is the wordplay.
'altered' indicates an anagram.
'moon' with letters rearranged gives 'MONO'.

Can you help me to learn more?

(Other definitions for mono that I've seen before include "Not stereo" , "Single-speaker sound" , "Transmitting over one channel" , "Single-channel sound" .)

I've seen this clue in The Mirror.

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