In, say, Mandarin Chinese, there are four ways of making meaning from the syllable mah. Depending upon the pitch or intonation, the syllable means four different things. Is there a written character for mah? Or do the concepts expressed verbally by that one syllable have four different written characters?
They have different characters. Think of it like “sheep” and “ship”. To non-english speakers they sound very similar. But to you they mean very different things.
There are various methods of writing tonal languages. Chinese uses ideograms, aka Chinese characters. The drawback to that is the character doesn’t necessarily indicate the tone. It is, after all, an ideogram, not a phonograph. There are also other ways of writing Chinese. Pinyin is pretty popular for certain purposes, with a method of indicating tones. There is another method known as Bopomofo or Zhuyin Fuhao.
Vietnamese uses a modified version of the Latin alphabet, with special symbols on the vowels to indicate the tone. Navajo also uses a modified version of the Latin alphabet, but with the apostrophe as the tone indicator; the placement of the apostrophe informs the reader which tone to use. Hmong uses the Latin alphabet but six letters are used to indicate the tone of the word; the tone letters are those letters that cannot be the terminal letter of a Hmong syllable.
You can check Omniglot for how the languages are written. Here is a world map of tone languages (you have to click on the “show map” button).
Just remember that even in English, the spelling is only a general guide to pronunciation and there are numerous examples of homonyms (words that sound identical but have different meanings) or homographic heteronyms (words spelled the same but pronounced differently, e.g. lead and lead). So perhaps the four different words pronounced ma could simply appear as is and leave context to disambiguate. English is a living example of the fact that orthography need be only a rough guide to pronunciation.
Pinyin has tone diacritics:
ā=high level
á=high rising
ă=low dipping
à=high falling
So:
mā=mother
má=hemp or linen or numb
mă=horse
mà=scold
Also, ma without a tone is used to mark a question. There aren’t many toneless words in Chinese ma? Which seems so odd, because in English rising question inflection would conflict with tonelessness?
In Hmong, they use a series of silent consonants at the end of a word, a different letter for each of the 8 tones. Since all Hmong syllables are open, they don’t need the consonants for anything else at the syllable coda, which frees up the letters to show tones. For example, the word for woman, poj niam, is actually pronounced “po nia,” with tones.
In Zhuang (a Tai language of southern China, related to Thai and Lao), they used to use glyphs adapted from the numerals from 2 to 6 stuck on the end of a word. But they revised their orthography and now, like in Hmong, use extra consonant letters stuck on the end of a word to show the tones. But the (I guess) most common Zhuang tone, number 1, the rising, isn’t marked; it’s just implied by the absence of an extra letter. You could even say paradoxically that it’s marked by being left unmarked. 
Pronounced “Son mwng hwn ma gva da,” with tones. W is a vowel in Zhuang, a vowel sound we don’t have in English, similar to Russian ы or Turkish ı.
When I had to learn some Hmong in college, I learned that some syllables ended in nasal consonants. Johanna’s example shows that.
Hm? Poj niam is the only Hmong example I gave, and there is no nasal sound in it except the syllable-initial n-. The name Hmong itself is Hmoob, pronounced /hmõ:/ with a high tone and a nasalized vowel. In English, the nasalization of the vowel is alluded to with the spelling “-ng.” When the vowel is written double, that shows it’s nasalized.
I was referring to the “Son” mistaking that post’s example for the Hmong example. At any rate, the fact remains that I was taught that in Hmong, some syllables do end in nasal consonants. It’s obviously been some time since I’ve had occasion to actually speak or read the lingo.
If each word had to be represented by a phonograph, writing would be an extremely slow and expensive business. 
Yes I know what you mean, and I expect you are technically using the word correctly. I just thought it was funny.
The tone of Thai syllables is controlled by consonant class and tone marks. Thus, since there are three consonant classes and five tone mark cases (one case being the absence of any tone mark), a 3x5 matrix mapping <class,mark> to one of five tones must be memorized.
For example, ไม่ (no) and ไหม้ (to burn) are pronounced identically – ‘mai’ with falling tone. In the first example word, the mark that looks like “1” provides the falling tone; in the second word, a silent consonant (ห) is prefixed just to change the syllable’s class, so a mark that looks like “2” is substituted to get the same falling tone.
The 3x5 matrix defining tones varies among the language/dialects related to Central Thai, implying, I think, that there used to be more than five tones, which merged differently in different dialects.
Vietnamese’s current writing system is, essentially, a big compromise. Words are spelled with consonants using the southern pronunciation and tone marks with the Northern pronunciation, regardless of how the dialect in use by a particular speaker actually is pronounced. This gives rise to the same actual tone in the Saigon dialect having two different tone marks.
Yes, in that example from Zhuang, the final -n in son is actually a real consonant, because it has the first tone which is not marked.
According to the Wikipedia article on Hmong vowel phonology, we’re both right. The same phoneme is pronounced either as a nasalized vowel or as a vowel with [ŋ] for a coda. The variance is not phonemic, so one can legitimately use either pronunciation. The time I listened to a Hmong instruction tape, the speakers pronounced it with nasalized vowels. But then the spelling “Hmong” itself supports using the -[ŋ] sound. In either case, Hmoob orthography allows the tone to be shown with a final letter, and the nasal vowel or -[ŋ] shown by doubling the vowel letter, at the same time.
Thanks for sharing that.
I looked up recordings, and spent a substantial amount of time fascinated by that vowel.
It’s like a ‘tr’ from train combined with the ‘ch’ from cheese. Neato!
The way I learned it was from G. L. Lewis: pronounce the first syllable in “cushion,” but instead of rounding your lips, spread them out flat. You have just pronounced the Turkish word for winter: kış.