Thank you Laza and Jeff for your replies. I understand thaty the folder based music handling can be complex. But I like it, because different photo topics require different styles of music and make a show more attractive. I use it also in digital AV shows with crossfaded photos syncronized to the beat of music.
What I like with Laza's skins, is the option for the end user to freely enable or disable some display features (camera data, navigation menu …). And I like possibility to get a simple style focusing on photos without additional colorful or distracting elements.
Due to this, I am sorry: I have to stay with Turtle. May be one day there is an updated (simple) turtle like skin with background music per folder? One nice feature would also be a photo file based start of next audio file, when a certain photo is displayed. For example photo1 audio1 photo2 photo3 … photo5 audio 2 … with every music or audio clip file is starting/repeating until the next audio file is reached.