Bug 22378: Fix audio alerts on SCO

Bug 22378: Fix audio alerts on SCO

Note: The SCO only supports sounds with a full URL, not KOha built in sounds

To test:
1 - Enable audio alerts
2 -  Enable SCO
3 - Note JS error on loading the alerts
4 - Apply patch
5 - No more error
6 - Test with bug 18251

Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit fb4707b97cec006e0e6f5c4b4dd430146e97f16a)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>