The songs are perhaps in the wrong format. This player will not accept copy-protected iTunes (AAC) files. Also, the license on your files may not permit transfer to mp3 players. Definitely check the bitrates on your files, b/c this Sony can only accept certain ranges of bit rate.
This is my general spiel about music formats, but note that your Sony does accept DRM-protected files with transfer rights, so not everything in the following blurb applies fully to you:
All of these problems occur because the user is trying to download a song format that the player does not recognize. Every music file is in a certain format. The most common music file formats are Mp3, WMA, AAC (iTunes), WAV, RA, etc. In addition, every file format type is in a certain bitrate, size, etc. By far the most common issue is that the music file contains a license or copyright (especially with WMA or AAC file formats). Each Mp3 player only recognizes a certain number of these formats. Every Mp3 Player is different. You will have to check your Mp3 player’s product specifications (specs) or user’s manual to find out which formats your particular Mp3 player recognizes. When you try to download or sync the wrong file format, you will get one of the errors that I mentioned above. If you want to know what file type you have, then you must find the location of the individual music file on your computer, right click the title of the song, and select the option “Properties” from the menu.Mp3 player product documentation is not straightforward. If a player supports only non-protected WMA files, it will merely say that it supports WMA – it will not tell which type of WMA it supports. A player that supports DRM-protected WMA’s will usually indicate such on the box (usually with a Windows Plays For Sure logo – which is actually an ironic misnomer). DRM stands for Digital Rights Management, and it is a type of licensing system for WMA files. It is used extensively by mp3 player music services such as Bearshare, LeapFrog, Napster, and others. Not all DRM licenses are created equal – there are licenses with “play rights,” burn rights,” and “transfer rights.” Play rights mean that you can only play the song on your PC – it will play fine on your computer, but it will not play in your mp3 player even though it appears to transfer. Burn rights mean that the song can be burned to CD. Transfer rights mean that the song can be transferred onto an mp3 player that supports DRM-protected files. Then, there are unlimited licenses and limited licenses. Limited licenses only allow you to play a song for a certain length of time. You would have to pay extra to continue using the song after that trial period is over – the time length ranges from a few days to several months or longer.
-Tha Mp3 Doctor