if ur receiver can see input status, will be clear.
possible got dolby prologic (emulated surround effect)

Since your optical to rca converter only support 2 channel output, the audio has obviously been changed by your AV Receiver. You don't have sub-woofer for LFE channel.

You also said that your AV receiver don't display input status so its not easy to find out why your receiver sometimes output 2 channel and sometimes 5 channels. Look at your receiver remote to see if there is/are any buttons that changes the 2 channels audio to emulate 5 channels. Usually most Denon, Marantz and Onkyo AV receivers have 4 buttons which are coloured in "Green, Red, Blue and Yellow" which can change the "Sound Mode" from original 2 channels to emulated (not true) 5.1 surround.

[attachmentid=7055657]

In my case, when I play a video with only 2 channel audio, I can press the remote "Green" Movie button to change from Stereo to Dolby Surround (5.1).

Since your AV receiver is a different brand, I don't know whether your receiver supports such features or not. If it does support such feature, your receiver may do it in a different way and probably you may have pressed one of your remote button which caused the 5 channels emulation.

If there is a Manual, you can check it to find out. You can also post a picture of your remote here to show the buttons that it have so that I can at least "guess" the reason.

From the photo file name you have shown here, its Audiovisio AV8815 Digital Karaoke HiFi Amplifier and I could not find any info on this Amp model using Google Search.

From your photo, I cannot identify all the amp buttons that are on this amp so without any more detail info on this amp's functions, I will not be able to find out the reason... and at the most, I cannot even make any guesses too. Unfortunately you do not have the remote and the remote on the photo is too small to see too.

By using the "Audiovisio" as the detailed search name, I suspect that this amp was made by a local Malaysian company, Audio Phonar Electronics Sdn. Bhd. However, I am not sure about this. Mmm... I suspect that its not made in Germany but I may be wrong.

Below is the link for a different model...
http://www.audiophonar.com/index.php?optio...=property&id=80Link to AudioVisio Home Amp

Here is another AudioVisio Digital Karaoke Amp posted in Mudah.my at...
http://www.mudah.my/Amplifier+AudioVisio+D...e+-46093466.htm
... and unfortunately the buttons identification are also blur.

You can contact Audiophonar at their website "http://www.audiophonar.com" IF you wish to find out more. Hehe... I will let you do all the work yourself to find out whether they sold this model or not AND what is the reason that caused the 5 channel output.

Your amp is a Karaoke HiFi unit so I do believe that there are functions on the amp which changes the Vocal Keys to slow and fast AND also can turn a Stereo source to Surround 5 channels. Although I am guessing, this is what I believe so a press of a button or a turn of a control knob on your amp can change this.

Probably you can take a clear photo of the front face of your amp to show the printed words for all the knobs and buttons which look similar to this below...
[attachmentid=7070111]

I know that I did not help much because of lack of any detailed info. This is as far as I can do to help and only giving my guesses.

From your close-up photos, there's completely nothing to indicate audio change switch selection and nothing about 5 channel audio output too. The only thing that I can only guess is the round "Echo" knob which can be used to adjust the reverberation effects of the audio. This "Echo" knob probably indicate that this amp creates a virtual 5 channel (with echo) by default. Most of the time this should be working properly to 5 channels but an internal fault in the amp decoder causes the output to become only 2 channels intermittently. Anyway, this is my wild guess only.

Well... I am still at a loss on the actual reason so I will just stop here as I can no longer do anything further to help you to find out the actual reason. I did my best and sorry for that.