Booming sales of its Walkman line of music phone doubled Sony Ericsson’s profits in the second quarter and boosted the vendor’s handset shipments by 33%, making the joint venture a contender for the fourth slot in global phone sales.

Sony Ericsson sold 15.7 million handsets in the quarter ending June 30, compared to 11.8 million in last year’s second quarter and 13.3 million in the first quarter. Those increased shipments were reflected in a 40% boost in revenues year-over-year to EURO 2.27 billion (U.S. $2.88 billion), and earnings increase of 91% to EURO 143 million ($181 million). Sony Ericsson credited new shipments of its growing line of Walkman music and Cybershot camera phones, both products based on Sony technology and brands the joint venture has successfully transferred to the mobile space.

The boost in units ship might send Sony Ericsson up one ring of the vendor ladder, depending on what No. 4 manufacturer LG Electronics reports later this month. In the first quarter, Sony Ericsson had 5.9% market share, while LG had a 6.9% share of 15.6 million phones shipped, according to IDC. LG, however, appears to be on a downward trajectory with shipments falling, while Sony Ericsson has been beating its own results quarter after quarter.

Though Sony Ericsson’s high-end camera and music phones seem to be the catalyst for the vendor’s huge growth curb, its average price per unit continues to fall. Sony Ericsson officials said that while they are continuing to focus on the high-end and mid-range tiers, the company has begun to ship much more entry-level phones in Asia and other developing markets.