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.
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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.