You must set it in System Setup, Alarms. The beeper works only when reaching the Marks with the Danger icon (the last one on the right in the Edit Mark page)

In order to avoid your marks and tracks during a software update or a ram clear, you need to save them in FILE MANAGER. Press twice the MENU key, select FILE MANAGER and press the ‘+’ key. Then press the ‘+’ key again. The File Save To Disk window is displayed. Once you have edited the name and the type fields, select OK and press the ‘+’ key to confirm. A Warning message is displayed. Select OK and press the ‘+’ key to save your marks or tracks.
To restore the mark rescued, enter FILE MANAGER press once the MENU key and select LOAD.  A Warning message is displayed. Select OK and press the ‘+’ key to restore your marks on the map.

In the Map+data mode page press the MENU key twice, select Setup data boxes, then press the ENT key. Select the box you want to replace with the Altitude one and press the ENT key. In the Data box fields select Navig:Altitude, then press the ENT key. To quit press the ESC key twice.