I found the solution :)
The problem is with parameter bgcolor in FlashVars. I've set this parameter to white (don't know if it can be 'transparent') and things are ok.
<object type="application/x-shockwave-flash" data="scripts/player_mp3_maxi.swf" width="340" height="20">
<param name="movie" value="scripts/player_mp3_maxi.swf" />
<param name="wmode" value="transparent" />
<param name="FlashVars" value="mp3=../tm_symbols/Himn_na_grad_Lyaskovets_online_128.mp3&width=340&autoload=0&volume=100&showstop=1&showvolume=1&showloading=always&volumewidth=50&volumeheight=10&loadingcolor=FFFFFF&bgcolor=FFFFFF&bgcolor1=73BE66&bgcolor2=73BE66&slidercolor1=FFFFFF&slidercolor2=CCCCCC&sliderovercolor=FFDB6C&buttoncolor=FFFFFF&buttonovercolor=FFDB6C&textcolor=FFFFFF" />
</object>