In your /etc/fstab how do you mount /media/sonnet/Main/oracle/ ? In other words, you should see some sort of line within /etc/fstab that may look like this:
/dev/sda$ /media/sonnet/Main/oracle ntfs defaults,umask=007,gid=XX 0 0
Where the $ in sda (or hda) represents the drive partition for your Windows drive and XX in the gid= represents the Group ID for 'sonnet'. the umask permissions are the allowed permissions for 'sonnet' when the drive is mounted.
In the example shown, the user and group have full read, write and execute permissions on the drive, while others are denied permission.
One other thing to note here in your statement:
chmod -R /runinstaller
This is incorrect, primarily because it would be looking for the runinstaller file on the root path. You have potentially made everything in root (/) world readable, writable and executable with that command. What you really wanted to do there was likely:
chown sonnet:sonnet /media/sonnet/Main/disk2/oracle/runinstaller
Then make the file executable by your user and group:
chmod 770 /media/sonnet/Main/disk2/oracle/runinstaller
Which should allow you to run that file (provided the partition is properly mounted) as user 'sonnet'.
Do HTH. Have a nice day. :)