Research on the effectivity of PowerPoint presentations as an adjunct to theoretical and practical content during university lectures has garnered significant yet inconclusive findings. Specifically, how multimodal academic content should be organised to aid communication remains unclear. Legitimation Code Theory (LCT) introduces the concept of 'semantic waves' as an effective means to understand cumulative knowledge-­building practices by modelling how forms of academic content are unpacked and repacked to facilitate understanding. This study applied 'semantic waves' to better understand how academic knowledge is constructed in multimodal PowerPoint presentations and whether differences exist based on discipline, language of instruction and modes used. Seventy-two lectures from subjects taught in the L1 and English-medium instruction were examined across four disciplines at two Spanish universities. Results showed that slide content in the soft sciences developed semantic waves to a greater extent, particularly in L1 Arts & Humanities lectures. Limiting slide content to the combination of text and graphics appeared to constrain scaffolding strategies in the process of meaning-making, while video content was linked to semantic waving. Finally, some pedagogical implications based on these results are presented.