"sound system" meaning a mobile system, right?
Yes - in the UK Duke Vin would travel from London to Manchester or Birmingham (or wherever), and people would follow his 'Sound' - and this would have been before they were playing Jamaican-produced records so it was US r&b
In mid-fifties Jamaica the sound systems had overtaken the live jazz bands that were big beforehand, and they played US imports. And there was huge rivalry between system operators, hence the blank labels mentioned earlier. When rock and roll took over in the US, Jamaicans started making their own records as they favoured the older r&b.. and this home-grown stuff developed into ska etc etc.
Interestingly, Duke Vin referred to himself as a 'disc jockey' (who plays records) and not a 'deejay' (who talks/toasts over them)
the sound systems started small but became huge, playing bass frequencies at 30,000 watts (this figure means nothing to me, I just read it in Steve Barrow's excellent 'Rough Guide to Reggae )