Will your choice of music be allowed?
If booking performers for ceremony music, it is best to check with your clergy about what is allowed in the church or what is considered suitable.

Will your musician hit the right notes?
If you are going to use a piano or organ at the ceremony venue, find out what is the right one for your musician and also how often the instrument is tuned.

How will a band sound in the space you have booked?
Be wary if there are high ceilings.

What kind of public liability insurance does the band have?
Some venues will want to know before letting them perform.

Does your band or dj have their own lighting?

Is there sufficent time for rehearsal?
Make sure you allow enough time for the musicians to learn any new, specially requested pieces of music. They will have to order the sheet music and rehearse which will take time.
Make sure the musicians have a back up plan in case equipment breaks down or someone gets sick.

Are there any additional costs for travel?

Once a fee is agreed,what kind of deposit is required?

When is the balance due and what is their cancellation policy?

What are the musicians be going to wearing?
You can request black tie if your wedding is formal and you don't want them to look out of place.

Make sure everything you have discussed in writing. Make sure both parties have emergency contact numbers.