The path separators shouldn't be a problem when converting your dictionary to a JSON string.
Your sample doesn't show the type & initialization of your json
variable but you can obtain a JSON representation from your dict the following way:
NSDictionary* jsonDict = @{ @"background": @"file://localhost/var/mobile/Applications/6118A03F-345B-42D5-AC19-25F6D9AC4484/Documents/background.caf",
@"bgMusic": @"file://localhost/var/mobile/Applications/6118A03F-345B-42D5-AC19-25F6D9AC4484/Documents/bgMusic.caf"};
NSData* jsonData = [NSJSONSerialization dataWithJSONObject:jsonDict options:0 error:nil];
NSString* jsonString = [[NSString alloc] initWithBytes:[jsonData bytes] length:[jsonData length] encoding:NSUTF8StringEncoding];
NSLog(@"Dict:%@", jsonString);
This works fine here (including proper escaping for the path separators in the log line)