"乱弹的声音" can be translated as "the sound of haphazard or unskilled playing (on a musical instrument)" or more concisely as "the sound of random strumming/plucking" (depending on the context, especially if referring to stringed instruments).
A more literal and general translation could be "the sound of random playing," but the above suggestions offer more context-specific options. For example:
The sound of haphazard piano playing filled the room.（乱弹钢琴的声音充满了房间。）
I could hear the sound of random strumming on the guitar outside.（我能听到外面乱弹吉他的声音。）