You are right. There is certainly a relationship between hearing and spelling, but it's not clear that this is the reason for your daughter's difficulties. Since words are made up of individual sounds called phonemes, it stands to reason that if a child can't hear the sounds, she may have difficulty producing them.
Most good readers and spellers have the ability to analyze the individual sound elements in words, and then blend the sounds together to make words. This is called phonemic analysis or "decoding" (making auditory sense of the visual symbols). In fact, difficulty with phonemic analysis is at the root of most reading disabilities. Since the brain "learns" how to understand sounds at a very early age, hearing loss whether it is permanent due to structural or neurological problems, or temporary due to repeated ear infections, can distort the sounds as they are transmitted to the auditory centers of the brain. As a result, when a child who is reading or spelling tries to pull out the stored memory of that sound element, what she retrieves may also be distorted. Improving reading and spelling in such cases involves teaching (or re-teaching) the brain to hear the proper sounds or phonemes and the letters or letter combinations they represent.
We're not sure whether this is your daughter's problem, but the answer may be found in an analysis of her spelling errors. I would want to know if she makes errors involving vowel sounds. For example, she might spell the word little as lettle or lettl or even lettel. This would suggest that she has difficulty hearing the differences between the short /i/ and the short /e/ sounds. Active current hearing loss as well as early auditory distortions may be at the root of this kind of error. On the other hand, if her errors are found in the way the word looks rather than in the way it sounds, this tends to argue against a purely auditorily based spelling problem. For example, if she spells alligator as lagitator, or if she spells the sight word* the as eth, this suggests that the problem may be visual (having to do with the sequencing of the visual elements) in a word.
To be sure about what's at the root of her spelling problems, you'll want to have the LD specialist in the school take a look at her spelling patterns. This teacher will want to examine her spelling on informal writing assignments as well as on spelling tests. Many kids can memorize a word well enough to spell it on a test, but not be able to apply it in everyday usage. Also, even though you say your daughter is a pretty good reader, you probably will want to have the school do the same kind of analysis of her reading errors. If the problems seem auditorily-based, talk to the learning disability specialist about techniques that can be used to strengthen sound symbol associations necessary for effective use of phonemes.
* Some insights on Sight Words: Most children (without visually based learning disabilities) are able to spell words that are usually learned by sight, such as and, is, the, etc. (These "sight words" are short enough that a child usually learns them as a visual unit without having to analyze each sound every time she sees the word. They are often found embedded in longer words and so, once learned, make it easier for kids to read longer words).