Well, tried a different soundcard (sound blaster), got the play sound quality (and volume) back...
... but then found that the 'record sound' level was so low that even at 100% MJ registered too low to record either from a tape recorder or record deck. It's usually about 15-20% on my 'working' computer - which incidently has the same on board sound processor (AC97)- for a reasonable quality recording. Tried playing around with the 'Line In' volume levels without any effect whatsoever.
So, back went the motherboard and was duly replaced by a brand new one under guarantee (you were right, of course, KingSparta). The new one was installed and played well for all of thirty seconds and then degenerated to the previous sound quality when I changed tracks, the vocals sounding like the singers are gargalling with water and the overall tracks, MP3s in particular, sound very much like a cassette does when it's being chewed in the machine. OGGs tend to have the vocals much reduced in volume with what seems to be an echo, the bass is hugely distorted and the treble is massively enhanced.
Boy, am I having a bad day...
Anyone know of any issues with the Avance AC97 onboard sound and the (Gigabyte) CA-7VKML AMD Socket A processor motherboard (VIA KM266 AGPset)? I have no idea what most of that means, but my digging around the web seems to suggest that a few gamers are complaining about loss of sound and 'clipping' with similar combinations.
Right now, my plans for a simple MJ-based music system are in tatters and I think my partner is about to leave me for an environment unpopulated by random loud expletives!