There are the "Ringing Rocks " of Pennsylvania which are unmarked and give out bell like sounds , India has lots too. Then there was some bloke (will try to find the article) who noted that a marked rock he hit with a hammer , why the hell he tried it in the first place is intriguing , also gave out a sound , wow , would you believe it ?
Considering how many marked rocks are outcrops or earthfast and therefore not the most resonant it wouldn't appear likely they were chosen for their accoustic properties . The addition of a few rings would not affect the pitch that much either .
Accoustics have been investigated in relation to prehistoric monuments , Aaron Watson did do some tests at stone circles and found to his own satisfaction anyway that there were spots that had anomalous readings .
Steve Waller is interested in echoes and has noted that some rock art sites in the US also have an echo .This site http://rockartuk.fotopic.net/c936680.html found earlier this year in Glen Lyon also has one along with the obvious landscape importance and associated rock art . I reckon that was £5 well saved .