Sight-singing is prevalent in aural skill classes, where learners differ in experience and cognitive abilities. In this research, we investigated whether musical experience, level of study, and working memory capacity (WMC) can predict sight-singing performance and if there is a correlation between WMC and performance among some subgroups of participants. We hypothesized that more experienced students and those with a higher WMC might sight-sing better than those with less experience and lesser WMC. We also hypothesized that the relationship between WMC and sight-singing performance would be more salient for less experienced and less proficient sight-singers. We surveyed 56 subjects about their experience with music, assessed their WMC, and evaluated their performance on a short sight-singing task. The results showed that the age when students began learning music could predict sight-singing performance independently from the number of years of experience and the educational level, suggesting a possible developmental component to sight-singing skill. We also found a negative relationship between WMC and pitch score in the low-performing group and between rhythm and pitch score, suggesting that pitch and rhythm are processed differently. Teachers should be aware of how students' backgrounds might be related to performance and encourage them to develop strong automated skills, such as reading music or singing basic tonal patterns.