Make your audio files smaller than 10MB -- the maximum file size for MP3 files uploaded to Tumblr -- using iTunes on your Windows or Mac computer.
Open the iTunes menu and select Preferences.
Click the Import Settings button.
Select MP3 Encoder from the Import Using drop-down menu and Good Quality (128kbps) from the Setting menu. Click OK to save your new import settings.
Select the audio file to convert from your iTunes library.
Open the File menu, hover your mouse over Create New Version and select Create MP3 Version.
ITunes then automatically creates a duplicate file using the quality settings you specified earlier in the Import Settings window. Right-click the new file and select Show in Windows Explorer -- or Show in Finder on a Mac -- and check whether the file size is under 10MB.