integer   gWasFlying;             
string    gSound = "footsteps";    
default
{
    state_entry()
    {
        
        llSetTimerEvent(0.25);
    }
    timer()
    {
        integer NowFlying = llGetAgentInfo(llGetOwner()) & AGENT_FLYING;
        if  (NowFlying != gWasFlying)        
        {
            llStopSound();
            if (NowFlying)      llLoopSound(gSound, 1.0);
        }
        gWasFlying = NowFlying;
    }
}