At level 7 the dual-modality span becomes genuinely difficult. The visual and auditory sequences are long enough that serial rehearsal fails — you cannot simply repeat them internally fast enough.
The strategy that moved me from a consistent ceiling of 7 to breaking through to 9:
1. Group digits into pairs. 4-7-2-9 becomes "47" and "29." Two chunks instead of four items.
2. Assign spatial positions to auditory sequences. I visualize the digits placed on a clock face.
3. Do not subvocalize the visual sequence. Look, encode spatially, move on. Subvocalization is too slow.
This is not a trick. These are well-documented working memory strategies from the cognitive science literature. The institution's ramp-to-failure structure forces you to find them because the ceiling is real.