I changed lines 108-110 to:

s+='#play_btn_up { position:fixed; right:0; bottom:90%;z-index:'+zIindex+'; height:136px; width:100%; cursor:pointer; background:url('+img_up+') no-repeat scroll 50% 50% rgba(0, 0, 0, 0.7); border-radius:5px 0 0 5px; margin-top:-24px; }'; 
s+='#play_btn_dn { position:fixed; right:0; top:90%;   z-index:'+zIindex+'; height:136px; width:100%; cursor:pointer; background:url('+img_dn+') no-repeat scroll 50% 50% rgba(0, 0, 0, 0.7); border-radius:5px 0 0 5px; margin-top:-24px; }'; 
// button class
s+='.play_btn { -webkit-transition-duration:0.5s linear; -o-transition-duration:0.5s linear; -moz-transition-duration:0.5s linear; transition-duration:0.5s linear; opacity:0.01; }'; 

Now the effect is more subtle and more friendly (at least to me). Thanks for the great work!