My computer isn't recognizing/mounting CDs and DVDs

With sudo lshw -C disk I get

  *-cdrom
       description: DVD-RAM writer
       product: DVDRAM GU71N
       vendor: HL-DT-ST
       physical id: 0.0.0
       bus info: scsi@1:0.0.0
       logical name: /dev/cdrom
       logical name: /dev/cdrw
       logical name: /dev/dvd
       logical name: /dev/dvdrw
       logical name: /dev/sr0
       version: 1.01
       capabilities: removable audio cd-r cd-rw dvd dvd-r dvd-ram
       configuration: ansiversion=5 status=ready
     *-medium
          physical id: 0
          logical name: /dev/cdrom

And with sudo mount /dev/sr0 /dev/dvd I get:

sudo mount /dev/sr0 /dev/dvd
mount: /dev/sr0 is write-protected, mounting read-only
mount: wrong fs type, bad option, bad superblock on /dev/sr0,
       missing codepage or helper program, or other error

       In some cases useful info is found in syslog - try
       dmesg | tail or so.

And when I once hit the Esc key during startup - with the DVD in the tray - one of the boot messages read:

I/O buffer error in /dev/sr0 at sector 4096

Does anybody know what the problem is?

Update 1: I have done the stuff suggested in How to install all existing media codecs? - only two additional packages were needed. So my computer was already up to date in this regard.

But I now attempted sudo mount /dev/sr0 /dev/dvd again, followed by dmesg:

[  519.520092] sr 1:0:0:0: [sr0] FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
[  519.520102] sr 1:0:0:0: [sr0] Sense Key : Illegal Request [current]
[  519.520109] sr 1:0:0:0: [sr0] Add. Sense: Read of scrambled sector without authentication
[  519.520113] sr 1:0:0:0: [sr0] CDB: 
[  519.520116] Read(10): 28 00 00 00 04 00 00 00 02 00
[  519.520131] blk_update_request: I/O error, dev sr0, sector 4096
[  521.020285] sr 1:0:0:0: [sr0] FAILED Result: hostbyte=DID_OK driverbyte=DRIVER_SENSE
[  521.020297] sr 1:0:0:0: [sr0] Sense Key : Illegal Request [current]
[  521.020304] sr 1:0:0:0: [sr0] Add. Sense: Read of scrambled sector without authentication
[  521.020308] sr 1:0:0:0: [sr0] CDB: 
[  521.020311] Read(10): 28 00 00 00 04 01 00 00 01 00
[  521.020328] blk_update_request: I/O error, dev sr0, sector 4100
[  521.020335] Buffer I/O error on dev sr0, logical block 1025, async page read

I think "scrambled sector" refers to the CSS encryption that exists on DVDs - do I still need to install something?

Update 2: Using VLC and its "Open Disc..." dialog, now it works. And this:

sudo mount /dev/sr0 /dev/dvd
mount: /dev/sr0 is write-protected, mounting read-only
mount: mount point /dev/sr0 is not a directory
dmesg | tail
[ 1378.434161] UDF-fs: INFO Mounting volume 'PLATOON', timestamp 2000/09/24 06:26 (1000)

...but yet, the DVD doesn't appear anywhere. I can't navigate to the DVD.