Is it possible, when creating a magic item to replace a command word, for say a wand, or a flaming longsword, to react to a fingersnap or similar sound instead of an actual spoken word?
The rules for Command Words state (emphasis mine):
Command word activation means that a character speaks the word and the item activates. No other special knowledge is needed.
Since command words require a character to actually speak them, the sound of a fingersnap or a sword coming out of its sheathe cannot be used to activate them.