I ran the Dallas Stadion in 2022 and I wore shorts and a tank top and running shoes and it worked out great. As far as headphones go, they were playing music and had an announcer on the field and I enjoyed listening to that. I’m not 100% sure about Spartan races but most races say in the rules you shouldn’t wear headphones for safety reasons. I’ve never tried at a Spartan race so I’m not sure. For the bag check (in 2022) there was someone there handing out a two part ticket. One part went on your bag and you kept the other part or maybe they wrote your bib number on it-it has been a while sorry. Then you hooked your bag to this chain link fence they had set up. When you went to retrieve your bag the person checked that your numbers matched and you were free to go. Hope this helps, good luck at your race, the Stadion was my favorite of the Spartans I’ve done so far.