Uploading MP3 files directly to Tumblr is probably the most convenient way of creating audio posts, provided you do not exceed the upload restrictions. If you do, there are alternative ways to make audio posts, so you're not left high and dry. Those alternatives do require a bit of planning, though, so knowing what the restrictions are and whether you're going to run into them in advance can be very helpful.
There are two different types of restrictions applied to uploading MP3 files. The first is one of file size -- uploaded MP3 files can only be a maximum of 10MB in size. If your file is bigger than that, you'll receive an error message when uploading. The second restriction concerns the number of files you can upload. As of December 2013, you can only upload a maximum of five MP3 files per day. This limit is account-wide, not per blog; all audio uploads count, even if you save the post as a draft or add it to the queue instead of publishing it immediately. Once you hit the limit, trying to upload another audio file will result in an error message.