A B737-300 Captain reported he failed to properly set the stabilizer trim prior to takeoff and got a warning horn as he advanced the power for takeoff. Apparently a distraction during cockpit setup contributed to the event.

Narrative

During the accomplishment of the Before Pushback Checklist; the Flight Attendant brought in the passenger count documentation at exactly the moment the First Officer read the 'takeoff trim' item. I responded to the Flight Attendant interruption and subsequent verbal exchange and then the First Officer and I proceeded to the next item; 'cockpit door;' without actually having reset the takeoff trim to the correct setting. During the takeoff from Runway 1R in LAS; I received a takeoff warning horn as I advanced the throttles for takeoff. At approximately 10 KTS; I rejected the takeoff and accomplished the immediate action items while the First Officer notified the Tower of the rejected takeoff. After clearing the runway and finishing the checklist items; I discovered the takeoff trim was not set in the proper position and was out of the green band area.This event reminded me to be extra vigilant of the impact of distractions during checklist accomplishment. In fact; it took several errors in procedure to arrive at the runway without the trim set properly. I am consistent about doing a throttle burst to check for incorrect items and could swear I accomplished one on this flight; but I either did it incorrectly; did not hear the very weak takeoff warning horn in this old -300; or failed to actually accomplish one. I would recommend that there be some minimum volume of horn that is acceptable. This particular -300's warning horn is quite weak and combined with other noise may have been masked.