CAIRO, March 19 (Reuters) - The Islamic State militant group has claimed responsibility for an attack on a museum in the Tunisian capital on Wednesday which killed 20 foreign tourists, according to an audio recording distributed online. It praised the two attackers whom the recording said were "knights of the Islamic State" who were armed with machineguns and bombs. (Reporting By Omar Fahmy; Writing by **ah Browning; Editing by Toby Chopra)