Mastering is currently limited to songs. At this time we don't support mastering for DJ Mixes (you actually want normalization) or Podcasts.
While SoundCloud accepts more audio formats than any other platform, mastering is limited to WAV (16 and 24 bit), FLAC, AIFF and MP3. For the highest fidelity masters, we recommend you upload in a lossless format. Finally, we do not currently support bit rates over 96000 kHz.
Mastering is currently limited to tracks under 10 min in length. If you're working on a long orchestra piece or just have a jam session that's a bit longer, drop us a note and give us a sense for what you're trying to accomplish and we'll consider it!