I believe you are referring to Input Monitoring. Band-in-a-Box is designed to create backing tracks and accompaniment for your chord progressions and personal song ideas. Input Monitoring is not supported in Band-in-a-Box. You will want to use a DAW program for this feature. Mac computers come with a DAW program pre installed called GarageBand that is far more suited to preform tasks such as Input monitoring.

Also if you are using a audio interface when recording your vocals you may have a input monitoring function directly on the interface hardware. If you are indeed using an audio interface please review the features of the interface as you may be able to do input monitoring in Band-in-a-Box this way with out using a 3rd Party DAW such as GarageBand or Logic Pro On the Mac platform.

Please still keep in mind Band-in-a-Box is not a DAW software and only allows one additional audio track to be recorded therefor is not ideal for tracking vocals.