By Eric Onstad LONDON (Reuters) -Sigma Broking Ltd became the first new ring trading member of the London Metal Exchange in 15 years on Wednesday amid doubts about the future of the ring and as the exchange seeks to bolster flagging volumes. The LME, the world’s oldest and largest market for industrial metals, said in a notice that it had approved Category 1 membership for Sigma. Sigma, which trades equity, fixed income and commodities, joins eight other banks and brokers that trade on the last open-outcry trading floor in Europe. The LME, owned by Hong Kong Exchanges and Clearing Ltd., reopen…