iPod My two 1st gen iPod(2001) always lag when playback bars in 2:20

[Magician_Alex]

Hi, everyone
I found my two 1st gen iPod(2001) in last day.

I tested to input and play some music in these devices.

And they are always lag when playback bars in 2:20.

According to a test of music format for my two 1st gen iPod,

Do not have this problem when playing MP3 and AAC format

The problem is happening on playing WAV and AIFF format in 16bit/44.1kHz.

Could anyone help me to explain and solve this problem?

Thank you