I have music in the WMA format on a USB key. I cannot copy songs in iTunes on my Mac and no error message is displayed.
Change the import options:
Select iTunes> Preferences, click Advanced, then Import.
Select an encoder from the pop "Import Using".
iTunes and iPod models equipped with a dock connector allows you to play songs in AAC or Apple Lossless. If you plan to listen to your music using another program or via an MP3 player, select MP3 Encoder.
To burn high-quality audio CD with songs imported without loss of quality, choose Apple Lossless or AIFF.(Remember that imported songs in this format occupy much more space.)
If you intend to listen to your music on a computer without software MP3, WAV choose.
Choose a speed from the pop-setting (not available with the Apple Lossless Encoder Encoder). In most cases you can keep the default configuration.
Low quality: Choose this setting if you choose MP3 encoder and if you plan to create your own audio CDs or listen to music on your stereo speakers high quality.
High quality: Choose this setting if you listen to music in a noisy environment. This setting creates files of approximately 1 MB per minute of music.
Good quality: Choose this setting to be able to put more songs onto an MP3 player with a limited storage capacity.
Customize: Choose this setting to have more control over file size and sound quality.