Implement the possibility of adding custom background noise in target format for each item

The item-generation scripts support adding low-level (+-4) random noise to all generated items via the .yml file using:

### Add low-level random background noise (amplitude +-4) instead of silence; default = false (silence)
add_low_level_random_noise: true

However, there is currently no option to specify a custom background noise file for individual items.