The Sydney Opera House is one of the most iconic music venues in the world. And yet it was also known for less-than-perfect acoustics in the main concert hall. The sound was considered thin and scattered. The problem has taken two years and 150 million Australian dollars to fix, involving 174 tonnes of steel in the roof space alone.Regina Botros joins a team of experts as they enter the final stages of re-tuning the building, ready for an opening night concert where everything has to sound just right.Presented by Regina Botros.Executive produced by Stephen Hughes for the BBC World Service.