i found a solution :
Reports have been flooding the web that the network transfer speed of Windows Vista drops to a low level when the user plays music at the same time. While there does not seem to be a obvious connection between network transfers and sounds being played and everyone would probably consider other possibilities first it seems that a service called MMCSS, the Multimedia Class Scheduler Service.
Mark Russinovich described the technical process in his blog explaining that ‘when a multimedia application begins playback, the multimedia APIs it uses call the MMCSS service to boost the priority of the playback thread into the realtime range’ which basically means that every other thread - including network transfers - see a performance drop because of this.
The following method has been working for most users who experienced the slowdown in network traffic in Vista while playing music.
Don’t play music while transferring files. This is pretty obvious but probably not the best solution for most users
Remove the MMCSS dependency of Windows Audio in the registry. To do that navigate to the key HKEY_LOCAL_MACHINE \SYSTEM \CurrentControlSet \Services \Audiosrv, right-click the DependOnService string and remove MMCSS from it.
Solution @ Fix for slow Vista Network Transfer while playing Music
The fix involves editing the Registry to turn off Network Throttling. As always create a System Restore Point to backup your Registry, then follow the steps below.
Click the Start Orb and type Regedit in the search box.
Click Regedit.exe at the top of the search results under programs.
if you are prompted for an administrator password or for a confirmation, type the password, or click Continue.
Navigate to the following key
In the right hand pane double click NetworkThrottlingIndex and type in FFFFFFFF (that’s 8 F’s) in the Hexadecimal field.
Click Ok and restart your computer for the settings to take effect.
@ How to Improve Network Performance in Windows Vista | Vista Revisited