Hi,
In iChat and the first version of Messages there was a Preferences pane that allowed you to change the sounds linked to specific actions.
See the top Pic on this Page
In Mavericks and Yosemite this item has disappeared and the only one you can change is the Messages Received one and this has been Moved to the General Section of the Preferences.
You have three choices.
1) Turn off the Play Sound option in Notifications
2) Use the System Preferences > Sound > Sound effects volume control to balance Alert Sounds against the Master volume.
3) Replace the sound in the app with a Silent one or one you like.
Open Quicktime.
Either record (File Menu in Quicktime) a new silent sound or Open and sound file you do like.
Use the Duplicate option in Quicktime (Also File Menu)
Rename the newly duplicated version to the "Logging In.aiff" like the one in Messages.
Save the item.
Then inside the Messages app again change the name of the current Logging In sound file (Add the word Old before the .aiff)
Leave this inside the App.
Drag the newly created one made in Quicktime to the Application.
It is most likely going to ask you to Authorise this with your Admin account's Password.
7:58 pm Friday; January 23, 2015
iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad