MP3 files don't play correctly in MM 3

[The Crow]

If I want to play mp3 files with MM3 RC4 (using standard input plug-in in_wmp3.dll) the file turns grey and the player jumps to the next file.
No problems with wav files, it also works with mp3 files using MM 2.5.

Then I tried plug-in in_mpg123.dll (2.07) instead of in_wmp3.dll. There it's the same with the difference that the files don't turn to grey but the player although jumps to the next file.

Then I installed in_dsmp3.dll (from MM 2.5). The mp3-files are played correctly but starting the track and doing position changes while playing always take about one or two seconds ("Seek" appears). Playing wav files doesn't take as long.

How can this be? In MM 2.5 everything works with the same plug-ins on the same computer.
I actually don't have these problems with MM 3 on another computer.
When changing the plug-ins I always renamed the old ones so there can't be a correlation.