Did you try the options in:
File Manager PCManFM > Edit > Preferences > Volume Management > Auto-mount
Unlike all previous versions, 7.6 doesn't automatically mount CDs. I have to type "mount /media/sr0".
Did you try the options in:
File Manager PCManFM > Edit > Preferences > Volume Management > Auto-mount
That doesn't help.
Ok, found the problem. You have to add kernel option:
block.events_dfl_poll_msecs=1000
Looks like a strange command to me but if it works ...
Looks to me like more Linux gibberish. Sure, Linux experts enjoy saying to noobs "you just have to add a kernel option". It certainly demonstrates their superiority and better understanding. But many average users have no idea exactly how you "add a kernel option". It isn't hard to understand why Linux is still marginalized on the desktop.
---
Verifying of md5 checksum and burning a CD at slow speed are important.
At least it's an easy fix, just add it to syslinux.cfg (assuming your knoppix is on a USB stick).
My Knoppix 7.7.0 is a LiveUSB on a Dell Inspiron which has a r/w cd/dvd drive in it.
This implies legacy boot and a remote possibility of systemd messing with things.
I was surprised that cds were not always being recognized, along with other media.
However, I finally noted that, for my set-up anyway, a cd physically in place
in the cd drive reader during boot will be recognized, whereas a cd added after boot
will not be recognized.
So, one work-around for Knoppix 7.7.0 LiveUSB legacy boot users is simply just to
re-boot with the cd in place. I would not be surprised that this problem and work-
around might apply to earlier Knoppix LiveUSBs as well, but I haven't checked this out.
Dell PowerEdge R730 2x E5-2699V3 2.3Ghz 36 Core 128GB RAM H730 X520-I350 2x750W
$329.99
Dell PowerEdge R630 Server 2x E5-2680 V4 = 28 Cores S130 32GB RAM NEW 480GB SSD
$197.99
1U Supermicro Server 10 Bay 2x Intel Xeon 3.3Ghz 8C 128GB RAM 240GB SSD 2x 10GBE
$259.00
Dell Poweredge R730xd LFF 14-Bay 2U Server | Choose Your CPU & RAM Config
$489.99
Dell PowerEdge R630 Server 2x E5-2640v3 2.60Ghz 16-Core 64GB H330
$182.65
Dell Poweredge R630 Server 2x E5-2620 V4 =16 Cores | S130 | 32GB RAM | 2x trays
$159.99
Dell PowerEdge R720xd 26HDD 300gb 2.5-inch E5-2697 X 2CPU 384RAM 7.2 Tb HDD 
$180.00
Dell PowerEdge T620 8-Bay LFF Xeon E5-2660 0 2.20GHz 48GB NO HDD S110 Server
$174.99
1U Server ( 4x VM's ) Hosting VPS Short Depth 20" 1U Xeon 3.5Ghz 16GB X10SLM-F
$149.00
DELL PowerEdge R630 8SFF Server 2x E5-2690v4 2.6GHz =28 Cores 256GB H730 4xRJ45
$562.00