The horn voltage from the D192C or D192G is draining down to 4.8 VDC after 40-50 seconds.
If using a D192C card, make sure that the "EXT PWR IN" is jumped to alarm trigger or run separately to battery positive. Alarm trigger will provide 2.0 A of power. The D192C card will allow up to 1.8 A of power max.
If using the D192G card, make sure that the "EXT PWR IN is jumped over to "AUX PWR" terminal on the card. The D192G card must use a constant supply of power to the "EXT PWR IN" terminal. If alarm trigger or battery positive is used, then a trouble condition will be present on the card. The D192G card can handle up to 3.0 A of power. The 7000-9000 series panel provides 1.8 A of aux power, which the panel uses 0.4 A for internal operations. This leaves 1.4 A for aux devices. In order to get the full 3.0 A of power, an external power supply must be used.