ICS may habve changed this option but normally you have to go to the messaging screen and activate the notification option.
Open gMail > Left menu button > more > Settings > Scroll to notification area at bottom of list...... All these options works for gMail
Messaging >Left Menu Button > Message Settings > Email > Notifications........ All these control all email accounts.
Messaging >Left Menu Button > Message Settings > Text Messaging > Scroll to notification area of list........ All these control all SMS and MMS accounts
Must be changed in ICS. You would think the Ringtones folder would be the default location for all ringtones, e-mai, messaging, etc. I really don't care for the default ringtones and I would think this new phone and ICS would have a option to add custom ringtones for text messaging, email.