Marilyn,
If I remember correctly, to use those speakers they need to have their own independent volume control. If memory serves me right the master volume on the PSR-740 will only control the onboard speakers and not external, which in a way works quite well because many owners have commented on how they can use the onboard speakers as monitors..... I don't think your keyboard is broken. This is just another of of Yamaha's tricky things...

Squeak