First, thanks for all the comments and advice on this little problem.
Regarding the absence of sound when playing MIDI files:
Wicked, you were dead right, I hadn't realised that there was no built-in synth in SONARX1, and as soon as I loaded one, all was well. Thanks for that.
Regarding the somewhat disparaging remarks about "
low-end DAWs": in my opinion you may be confusing "low-end" with "higher-value", as I consider Mixcraft 5 to be a very good product, and it has most of the features of SONARX1 at a much lower cost. And I also consider the Acoustica Instrument set which is supplied with it to be amonst the best I have heard. If you are in any doubt please try them.
Regarding "getting a proper sound-card":
Plazma, I'm glad to say that I already have an M-Audio Delta 1010LT card in the PC. It was not mentioned because at the time I wasn't using it with SONARX1, but I have it, and I recognise the comments made - they were just a bit presumptive!
And
Plazma - I have 2 Strats, and a Gibson Les Paul eh! And a Yamaha baby grand! You can't really beat real instruments eh! Until you gat a Hauptwerk console that is. And a Terratec AXON (but now I want one of those new Roland Guitar synths).
Finally, all is now well I'm glad to say, so thanks again for the help! My real error was not failing to understand MIDI (as Kalle-Rantaho said) but simply missing a point about SONARX1.
All the best,
Ken