Hi to all.
 
I'm experiencing problem with exported audio.
 
When I export a audio, the audio has lower volume at the start, and restores to normal volume.
 
Why is this, and could you suggest me some solutions?
 
Thanks in advance.

You mean the exported one differs from the project, or are you asking how to balance the projects volume? 
There's no function or feature that could inadvertantly change the volume if there's no FX or automation inserted to cause it. 
 
So, if the exported one differs from the project, there must be automation, FX, missrouting or something that causes it.
Is everything routed through master buss?

One way to check if there really is an export problem is import the exported audio file into SONAR. If the playback is correct in SONAR, there is an issue with the other playback software. Some playback software "enhances" the sound, boosting bass or altering the dynamic range, resulting in exports sounding strange.

My playback program probably has fade in feature that causes this 'problem.'
I'll look for it, but that must be it.
 
Thanks for the tip.
Issue solved.

Get a copy of Wavepad or Audacity to use to listen to the exported waves.  They make no assumptions as to what you may or may not want. They simply play the wave just as it come out of the DAW.