As Erik suggested.. open the event editor and look for Vol entries there.... for example that could be interfering with things..
If you have an iPad or iPhone and have used any Logic Pro related apps.. turn them physically off, leave them off and try out Logic Pro again...
Next, if that doesnt fix things.. shut down your Mac and unplug any midi controllers you may have.... and then test again
Finally, try the true and much tested 'delete prefs' method...
http://support.apple.com/kb/HT5859
and specifically this part....
Delete the user preferences
You can resolve many issues by restoring Logic Pro X back to its original settings. This will not impact your media files. To reset your Logic Pro X user preference settings to their original state, do the following:
- In the Finder, choose Go to Folder from the Go menu.
- Type ~/Library/Preferences in the "Go to the folder" field.
- Press the Go button.
- Remove the com.apple.logic10.plist file from the Preferences folder. Note that if you have programmed any custom key commands, this will reset them to the defaults. You may wish to export your custom key command as a preset before performing this step. See the Logic Pro X User Manual for details on how to do this. If you are having trouble with a control surface in Logic Pro X, then you may also wish to delete the com.apple.logic.pro.cs file from the preferences folder.
- If you have upgraded from an earlier version of Logic Pro, you should also remove~/Library/Preferences/Logic/com.apple.logic.pro.
- Restart the computer.
Then test again