The answer depends on what level in the axilla you are scanning at.
I use the humeral head as a landmark.
If you can see the hyaline cartilage on the humeral head the following muscles are visible. Medially and superficially you see the pec major. Beneath this you see pec minor. Laterally you see latissimus dorsi.
If you are further out in the axilla where you are distal to the humeral head and scanning over the humeral neck you see different muscles. Medially and superficially you see the short head of biceps and underneath it you see coracobrachialis. Between these muscles you see the musculocutaneous nerve. Laterally you see the triceps muscles.
Steve