If this time you always hear this sound every login at ubuntu, may you will never find it in Ubuntu 12.04 Precise Pangolin, because it's will disabled by default - confirmed by jeremy bicha
the Ubuntu startup sound which have been used for 5 years will be disable by default in Precise Pangolin, on the latest Ubuntu 12.04 development update news, a new version of libcanberra has been upload with startup sound which is disabled.
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| Ubuntu system sound setting |