Lucian Williams
Thanks for the suggestions. None of them worked. I went to EDIT>PREFERENCES>PLAYBACK AND RECORDING AND SELECTED WDM/KS to the right of DRIVER MODE: I clicked "Apply" and a window came up - SILENT BUSES DETECTED, a paragraph under it yada yada....
Right. As has been said, the "silent buses" window is probably just warning you that you have at least one bus in the current project (probably the master) that isn't pointing at an audio out. Which is not surprising if you've changed driver mode. Now have a look in preferences audio devices and you should see the realtek listed, with tick boxes next to the list entries. Tick those boxes, hit "apply" and close preferences. If not listed try switching the driver mode to MME or wasapi. On-board soundchips can take a bit of work to get them operating properly with DAW software, and they'll never match an interface with a good ASIO driver in all sorts of ways, but they can usually be persuaded to give playback.
Then point the project's master bus output at the realtek which with any luck should now be listed in the bus output pull-down menu. Finally, check the windows volume control isn't turned right down. Sonar should bypass it, but with onboard sound-chips you never can tell.
As for the other problems you're having, let's go one step at a time with one problem at a time and see if between us we can work out a solution.
Ps - the specific contents of an error or warning message - the "yada yada" - might be important, so it's usually a good idea to quote them in full. The yada-yada might not always be the same. :-)