Best Buy Co.’s same-store sales fell 2.7 percent in the second quarter — worse than the 2.2 percent analysts forecast. Net income fell 45 percent to $146 million from a year earlier. And sales will decline by a ‘‘low single digit’’ percentage rate in the third and fourth quarters, the company said. CEO Hubert Joly has made good on a vow to cut expenses since arriving in September 2012, helping the stock triple last year. Since then, the shares have slipped as investors doubt whether Joly can revive sales growth.