Ever since Minecraft 1.3, block placement sounds have been controlled by the server rather than the client.
If the user's connection to the server has relatively high latency (~100ms+), the sound lag is very noticeable. This is confusing and also inconsistent with block breaking sounds, which are controlled by the client and will therefore be lag-free.
Please consider switching this back to the pre-1.3 behavior, allowing the client to generate block placement sounds immediately without having to wait for a server round trip.
This issue was reported a while ago in
MC-30, but was only partially fixed. There is also a client mod (of which I am the author) to provide a temporary workaround: NoSoundLag, http://www.minecraftforum.net/topic/1414774-nosoundlag/