What pins are to be connected to the SD card.
D4 - SS or CS
D11 - MOSI
D12 - MISO
D13 - SCK
First, hardware SS is often only an issue when SPI is used as slave, which is often not the case.
Yes, on the 328P it was working OK on 9 & 10 with SD Card used on SPI and with D4 as SS. On my new PCB's I could also be using radio and ethernet modules on SPI (I know they work together), as well as SimpleSDAudio, although any sounds would only be played when those devices aren't being used. I thought it would be cleaner if I could use 6 & 5 instead.
Second, to use the pins 6 & 5 on mega328 Arduinos, you have to apply the wiring.c patch as you already figured out and then you have to patch the file SimpleSDAudioDefs.h. Scroll to the end and you find the last big #else part that sets up all the stuff for mega328 Arduino. Below that, there is the same part again but this time for use pin 6 & 5, but it is commented out. Rework the comments that the old #else-part for pins 9 & 10 will become commented out and activate the part below instead. Then audio will use pin 6 & 5, but it requires the wiring.c patch. Beware, I have not tested that thoroughly so please leave a note if it works for you.
Patches all made and audio works perfectly now switched to Pins 6 & 5. Tested in 8 bit Stereo Fullrate. Many thanks again. :-)