Choose the Hi-Res Audio product type when creating a new product. Hi-Res Audio (HRA) audio assets must be 44.1KHz / 24 bit or 96KHz / 24 bit. The same quality must be uploaded for every track on a given release.
Note: Choosing and submitting HRA products significantly impacts distribution of the product. The vast majority of music services only accept standard audio product types. By choosing HRA, you will be limiting the product to distribution to HD Tracks and iTunes (if eligible as a "Mastered for iTunes" product).