Wikipedia lists "is sound only sound if a person hears it?" as "The most immediate philosophical topic that the riddle introduces."
This is the question that Eliezer addresses. You may have been introduced to the tree falling riddle in a different context, but he's dealing with the most common interpretation of the question.
You can't accuse him of straw manning simply because he didn't deal with some other possible philosophical question regarding trees and sounds (the wikipedia article lists a few others, including something like yours). He clearly states up front the exact argument that he's dealing with.
This is the question that Eliezer addresses. You may have been introduced to the tree falling riddle in a different context, but he's dealing with the most common interpretation of the question.
You can't accuse him of straw manning simply because he didn't deal with some other possible philosophical question regarding trees and sounds (the wikipedia article lists a few others, including something like yours). He clearly states up front the exact argument that he's dealing with.