If the 2500mAh batteries are AA NIMH then you'll need 5 of them to power your horn and they should run it for about 1hr 40 mins.

I don't know what the runtime on your torch should be, but you said batteries and judging by the capacity I guess it's 2 x AA NIMH.

If it is try changing the batteries, if one of the batteries is a bit tired it will effect the overall result.

If the batteries are a bit elderly, some new ones might perform a lot better.

Finally, if all your batteries seem to be underperforming, suspect your charger.