Tekoa is a town south of Jerusalem and Beth-haccherem is a hillside probably also to the south. Both the blowing of the trumpet and the raising of a signal (probably a fire that was lit) were used to alert people to impending danger form an invading force. The reason these two places are mentioned is probably because the invasion is coming from the north and people will have to flee in a southerly direction.1
1 Fuyez, enfants de Benjamin, du milieu de Jérusalem, Sonnez de la trompette à Tekoa, Elevez un signal à Beth-Hakkérem! Car on voit venir du septentrion le malheur Et un grand désastre.