To give a single, summary comment, you could alternatively:
All of these are free to UCL staff.
There are other technologies for giving contextualised audio comments i.e. attached to individual paragraphs and capable of more specificity. However, these either use proprietary technologies (Adobe Acrobat Professional) which cost money, or they do not work on every platform, or generate very large files.
Dr Chris Evans from UCLIC, who has researched audio feedback, recommends:
Chiang, I. A., 2009. Which audio feedback is best?: Optimising audio feedback to maximise student and staff experience. A Word In Your Ear 2009. Sheffield Hallam University, City Campus. 18 December 2009.
Crisp, B., 2007. Is it worth the effort? How feedback influences students' subsequent submission of assessable work. Assessment & Evaluation in Higher Education, 32(5), 571-581.
Emery, R. & Atkinson, A. 'Group Assessment Feedback: The Good the Bad and the Ugly'. A Word In Your Ear 2009. Sheffield Hallam University, City Campus. 18th December 2009. Available at: http://research.shu.ac.uk/lti/awordinyourear2009/docs/emery-atkinson-Solent_Audio_Feedback_paper.pdf .
Evans, C., and Palacios, L., 2010. Using Audio to Enhance Learner Feedback. In: Proceedings of the Education and Management Technology (ICEMT), 2010 International Conference. 2-4 November 201, Cairo. Pp. 148–151. IEEE. http://ieeexplore.ieee.org/document/5657683/.
Lunt T, Curran J. “Are you listening please?” The advantages of electronic audio feedback compared to written feedback. Assessment & Evaluation in Higher Education. 2010 Dec;35(7):759–69.
Merry, S. & Orsmond, P., 2008. Students' attitudes to and usage of academic feedback provided via audio files. Bioscience Education Journal, 11. Available at: https://www.reading.ac.uk/web/FILES/EngageinFeedback/Students_attitudes_to_podcasts.pdf .
Milanowski, T., 2009. e-Feedback for formative assessment: can it improve student learning and assessment efficiency? A Word In Your Ear 2009. Sheffield Hallam University, City Campus. 18th December 2009. Available at: http://research.shu.ac.uk/lti/awordinyourear2009/docs/Tony-Milanowski-E-feedback.pdf.
Nie, M. et al., 2010. The role of podcasting in effective curriculum renewal. ALT-J Research in Learning Technology, 18(2), 105 – 118.
Pachler, N., Daly, C., Mor, Y., and Mellar, H., 2010. Formative e-assessment: Practitioner cases. Computers & Education, 54, 715-721.
Rotherham, B., 2008. Using an MP3 recorder to give feedback on student assignments. Available at: http://sites.google.com/site/soundsgooduk/downloads.
Rust, C., 2001. A Briefing on Assessment of Large Groups. Available at:https://nursing-midwifery.tcd.ie/assets/director-staff-edu-dev/pdf/AssessingLargeGroups-ChrisRust.pdf