The WAV format is not mixed-endian; everything that’s actually a number is little-endian. The fields the diagram in the article marks as “big endian” are ASCII

The WAV format is not mixed-endian; everything that’s actually a number is little-endian. The fields the diagram in the article marks as “big endian” are ASCII text— four-byte strings with specific content (“RIFF” for the first ChunkID, “WAVE” for the Format, etc.). If you wanted to treat those as integers in your code, you’d need to treat them as big-endian so their byte order doesn’t get reversed, but they’re really four-byte sequences, not numbers.