Here are a few tips regarding latency issues with Mbox:
Tip #1:
When recording into Pro Tools LE:
1) Mute recorded enabled audio track in Pro Tools. When recording on Mbox, you are actually monitoring the audio signal going to Mbox. If the record enabled audio track is not muted, you will also be monitoring the signal going out of Pro Tools back to Mbox. This will be heard as a doubling effect because you are hearing both the input and the latent output at the same time.
2) Adjust the "Mix" knob on Mbox to prevent any echo or latency sound.
3) Press the "Mono" button on Mbox. This will prevent the input signal from sounding hard panned left or right when adjusting the Mix knob on Mbox. The "Mono" knob does not affect the mix playing out of Pro Tools, just the incoming signal.
Tip #2:
1) When you record into Pro Tools there is a latency of 164 samples. All tracks recorded into Pro Tools have 164 samples of latency, but you will not notice this with the first track unless you have a MIDI click, or a recorded click to play along with. Thus, when you record the second track/pass, you will hear latency when the two tracks are played together. Tip #3 explains how to deal with this issue.
2) Tip #1 just helps to prevent hearing the latency while recording into Pro Tools; however, there will be latency when recording into Pro Tools/Mbox.
Tip #3:
1) After recording your second track/pass into Pro Tools, adjust the newly recorded audio track to compensate for the 164 samples of latency.
2) Select the second pass of audio with the Grabber Tool. Edit Menu > Shift. Click "Earlier" and type in 164 in the "Samples" field. Click "OK". Now, Pro Tools will locate the audio 164 samples earlier.
3) "Shift" each track or recorded pass back by 164 samples immediately after recording. If you are stacking up tracks, you definitely want them all to be in sync as each new one is recorded.
4) If you recorded your audio at the very start of the session, you will have to trim in the audio before the waveform starts to allow space for the audio to be shifted 164 samples ahead.
Now, you may ask, are all other passes 164 samples apart from each other too?
Not exactly, each recording pass will be 164 samples behind everything already recorded. If you record one track at a time, and mute recorded tracks as you go along so you're only listening to the most recent track, then yes, your final track (say, 24) will be 23 x164 samples behind the first track, at least in terms of what you're hearing. Technically, the last track is still only 164 samples behind the first track but listening to these delays building up will cause your playing in time to the tracks to start falling behind noticeably.
So if you add one track at a time and listen to all previous tracks on each recording you'll be hearing:
Track 1: On time
Track 2: 164 samples behind Track 1
Track 3: 164 samples behind Tracks 1 and 2, so play in time to Track 1 you're off by 164 samples, but if you play in time to Track 2 you're 328 samples off.
Track 4: 164 samples behind Tracks 1, 2, and 3, but the delays build up differently depending on what track you play off of.