When submitting recordings that belong together, like movements from a sonata or a number or pieces from the same opus, it is desirable that you post one message with multiple attachments, rather than posting a new message for each recording. However it is not quite intuitive for many people how to do that, so here's some explanation.
Please refer to the attached screenshots upload.gif and posting.gif.
First, you create a new posting in the Audition Room, explaining what you are submitting. Please enter a title containing the name of the composer, work(s), and opus number(s). You enter yourtext in the message area - this is mandatory (the message area may not be left blank. Below the 'Message' and 'Options' areas, you see the area titled 'Add an attachment'. You first click the 'Browse' button to locate your mp3 file (or you type the filename if you know it). Optionally, enter a file comment. When you click the 'Add attachment' button, the system will be busy for a while, uploading the file, then present you with a new area to attach another one. It now shows a new area below this, titled 'Posted attachments' where you can see what files are already done. The screenshot upload.gif shows this situation after attaching two files.
NOTE Each attachment will be shown on top of the previous one. So if you care about the order in which they will appear, you should work in reverse order. For example if you post 3 movements of a sonata, attach them in the order 3 - 2 - 1. Is is not possible to change the order of attachments once they are created.
Once you are done with adding attachments, you click the Submit button (there are two on the screen, it does not matter which one you use). The forum will first tell you the message has been posted, and after some seconds it takes you to the new message message automatically. In this case, the message looks like shown in the screenshot posting.gif". Only if you see your posting like this, with the attachments, it has succeeded. Of course it can do no harm to check the attachments there and then by clicking on them. If something is wrong, you can click the 'Edit' button to change the message and/or the attachments.
I don't know whether I like it, but it's what I meant. -- Ralph Vaughan Williams, on his 4th Symphony