figures show that while millions still tune in they listen for shorter bursts .
average listener spent ten hours a week tuning in last three months of 2014 .
this was 14 % down on decade earlier , when people tuned in for 11.6 hours .
the bbc trust has cleared the way for firms to buy their way into lifestyle programmes on the world news channel in a product placement experiment . for example , publishers could pay to have their books reviewed on talking books . the bbc trust will review the scheme in a year .