This really depends on your engineer.Â  The common formats are all digital, but if youâ€re a guy who records to 2-in tape, thereâ€re engineers out there who work with it.Â  The general rule is, though, you want to give your engineer the highest-possible quality mxidown you can provide, in whatever format the engineer will accept.Â  So donâ€t send a maxed-out MP3 file.Â  Usually a 24bit, 44.1khz audio file in a standard, uncompressed format (wav, aiff, SDII) is sufficient.Â  Again, discuss this with your engineer, as each may have different requirements