Satellite radio company Sirius XM Radio Inc. said Tuesday that its second quarter profit soared as its revenue and subscriber base grew while expenses declined. The company reported net income of $173.3 million, or 3 cents per share, up from earnings of $15.3 million, or breakeven per share, in the same period a year earlier. Revenue rose 6 percent to $744.4 million from $699.8 million. Sirius ended the quarter with about 21 million subscribers, up 8 percent from a year earlier. Auto sales growth helped boost the numbers. The company expects to add 1.6 million net subscribers in 2011, up from its earlier guidance of an increase of 1.4 million. Sirius still expects full-year revenue of $3 billion. Analysts are expecting $3.03 billion.