When a human building in range of the DC is hit, the game qvm creates an event of type EV_DCC_ATTACK. In the cgame qvm, in file cg_envent.c this event is handled:
case EV_DCC_ATTACK:
if( cg.predictedPlayerState.stats[ STAT_TEAM ] == TEAM_HUMANS )
{
//trap_S_StartLocalSound( cgs.media.humanDCCAttack, CHAN_ANNOUNCER );
CG_CenterPrint( "Our base is under attack!", 200, GIANTCHAR_WIDTH * 4 );
}
break;
So apparently the devs intended to play a sound here, but the sound was commented out later (due to a lack of a sound file ?). So you just have to remove the comment markers and register an appropriate sound (modify cg_local.h and cg_main.h).