We generally do not listen to music in class. I will sometimes bring my Shuffle and listen while I am practicing by myself but I would not impose my music on anyone else.
Class is a collection of many people, all with different tastes and different, changing emotional conditions. Music carries emotional information, and can affect mood strongly and unpredictably.
I do not want to have to juggle musical tastes in an aikido setting, easy solution, have none. I cannot control others' emotional reactions to music, so it is easier to have none. Like Janet, I do not want my responses influenced by an irrelevant external source, better to have no music. Music is personal, I wont inflict my tastes on other people, that'd be rude.
And, when we've tried music in group training, some people got too jacked up and got injured. Or, class became a lovely little sitdown and chat on the mat session. Neither is much good for effective training.
The obstacles to the use of music in Aikido practice are certainly not trivial and I can understand why few aikidoka would try to change established traditions when there appears little to gain and much to lose in the attempt.
I believe your use of music in your preparation and solo practice is not uncommon. Your personal experience trying music in group training is, I think, valuable information as well and speaks to both your curiosity and willingness to explore the possible advantages to be gained from incorporating music into the group practice of Aikido. However, despite the obstacles stated, and despite the evidence pointed to that under the stated conditions it failed to yield positive results, I wouldn't give up on further exploration in this area.
I think you believe, as many others do: -- music has power and spirit all its own. That power can be disruptive socially as well as psychologically and has obvious potential to interfere with the spiritual nature of a group's Aikido practice and the sanctuary nature of the dojo.
My hope is that music might be productively combined with Aikido and that some dojos will continue to explore this relationship since there is great potential benefit-- both physical and spiritual -- in a connection of the two.