W25Q128JVSIM not working on Teensy 4.1 but on Audio Shield?

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I soldered a W25Q128JVSIM flash memory onto the Audio shield and it works fine.
I soldered another one to a Teensy 4.1 and I get "Unable to access SPI Flash chip" from "ReadBenchmark" example.
I tried it with two different Teensys with different flash memory chips. Always the same error.
I uncommented the Serial.printf line in function "readID" in file "SerialFlashChip.cpp" and it shows "ID: 00 00 00".
Is it possible that the W25Q128JVSIM flash chips are working on the Audio Shield but not on the Teensy 4.1?
When using an Audio Shield with addition flash you are actually using the flash chip in SPI (normal) mode. Which means you are using the normal SPI interface to access the chip.

When using the T4.1 with flash chip soldered on the bottom of the board you are interfacing the chip with FlexSPI and using what they call quad SPI which is different than just using SPI. The library you are using to access the bottom memory won't work with the chip soldered on the bottom on board.

To access the flash on the bottom of the T4.1 you need to use the SPIFFS_t4 library which has examples for using a SPIFFS file system or writing directly to the flash chip (flashtest6.ino): https://github.com/PaulStoffregen/teensy41_extram/tree/SPIFFS-FLASH-ONLY
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