Is it permissible to continue volunteer work collecting donations to buy toys and electronic devices for sick children, while fearing that they might contain music, and would the female volunteer incur sin in this case?

As long as the music in children's toys is not the primary or predominant purpose for acquiring them, then purchasing them is not deemed forbidden. This is in accordance with the principle: "That which is tolerated implicitly in something, is not tolerated explicitly when intended," or "A تابع (subsidiary) is not singled out for a ruling unless it becomes an intended purpose." Therefore, there is no impediment for the questioner to remain in her job. It is preferable to choose versions of toys that do not contain music, if possible.